In another thread (Prepositions) Chameleon made reference to the difference between "different from" and "different to". For what it's worth, here's my take on it. DIFFERENT FROM is correct. All else is ... less correct. Here's why:

Why is "different from" correct?

Okay, let's start with some logical reasoning. See - it all starts with the verb "to differ". This is an intransitive verb, so you can't say "A differs B". You can, however, use it reciprocally, as in "A and B differ from each other". "To differ" has two acceptable prepositional forms, each with a different meaning - "to differ FROM", and "to differ WITH".

1. To differ FROM something is "to be unlike or distinct in one or more respects or characteristics" or "be unlike in nature or form" - these definitions are from Merriam-Webster, which is of course an American dictionary, but the definitions agree with the British ones, so everyone's happy.
2. To differ WITH someone is to disagree with them - in the words of M-W: "to be of unlike or opposite opinion : disagree in sentiment".

In other words, if I differ FROM you, then I am not the same as you; if I differ WITH you, then I disagree with you. Both America and Britain are happy to agree on this usage.

Now, the word "different" is derived from "to differ" in just the same way that "conformant" is derived from "to conform", or "indulgent" is derived from "to indulge". So you might suppose that we ought to be able to say that to be different FROM something is to not be the same as it, and that to be different WITH something is to disagree with it. However, the latter meaning does not carry over into the derived form - this is because WITH already has a positional meaning, and so the sentence would have otherwise became ambiguous. (When we say "It's different WITH her", we mean "It's different when I'm with her". We can similarly say "It's different FOR you", meaning "It's different when it's you instead of me"). So this leaves us with only "different FROM" transferred from "to differ FROM".

Why is "different to" incorrect?

"Different to" is incorrect only in the sense that it does not logically fall out of the above argument. However, there is still some formal justification for it. It could be argued that "different to" is merely employing ellipsis - that it is an abbreviation for "different (compared) to", or "different (in relation) to", and so on. There are any number of phrases you could insert to make it make sense, and since doing so does not introduce any real ambiguity, it is hard to argue that this usage should be outlawed. Consequently, though the formal position remains that "different from" is the axiomatically correct version, "different to" is quite acceptable in spoken English.

Why is "different than" incorrect?

Quite simply because "than" is not a preposition at all - it is a conjunction. Compare these sentences:

1. I like him more than her.
2. I like him more than she.

Both are correct, but they mean different things. In full, they mean:

1. I like him more than (I like) her.
2. I like him more than she (likes him).

Clearly, the entire phrase "more than" is acting as a conjunction - this is evident from the objective case form (2). A preposition, on the other hand, would demand subjective case. (In fact, "different" may never take an objective case because it is intransitive). So "than" is a conjunction, and its correct use is in phrases such as "more than" and "less than".

Is it ever correct to say "different than"?

It can be. Both of the following sentences are sound. Sentence (1) is okay for fairly obvious reasons, but (2) might require some explanation.

1. I am more different than she.
2. The working day ends at a different time for you than it does for me.

Sentence 1 would not be formally correct, had it said "her" instead of "she". It is correct because THAN goes with MORE, not with DIFFERENT. We are in effect saying "I am more different (from something) than she (is)".

Sentence 2, surprisingly, is actually acceptable, perhaps because lots of respectable writers have done this sort of thing over the centuries. This usage is actually fairly simple to explain - here, "than" is being used as an abbreviation for "from that at which". In other words, the fully expanded sentence should be: "The working day ends at a different time for you FROM THAT AT WHICH it does for me." This version of the sentence is long and cumbersome - the version with THAN is smoother and easier to follow. In general, the rule is that "different from that ___ which" (where "___" is any preposition) may be replaced with "different than", even in British English. This does not, however, justify replacing "different from" with "different than" in all instances.

So why do Americans say "different than" all the time?

Who knows? Usage is usage. If they keep doing it for long enough, this usage will eventually become historical, and then eventually it will become formally acceptable. Usage drives the rules - if the usage is used consistently and for long enough. The American usage is by now well established - at least in America. It's not going to go away, and the Americans prefer to say "different than" over and above either of the other forms. So perhaps, in time, "than" will change its classification and become a preposition after all. Only time will tell.

Best advice? In formal writing, use "different from". In speech, use "different from" or "different to" in England, "different than" in America.

All three variations are allowed:

Function: adjective

1 : partly or totally unlike in nature, form, or quality <two men could hardly be more different> : having at least one property not possessed by another (of a specified pair or larger group) <no thing is different from itself>

-- used with from <small, neat hand, very different from the captain's tottery characters -- R.L.Stevenson>

or with than<different than any other piece we've done lately -- Harper's> <vastly different in size than it was twenty-five years ago -- N.M.Pusey>

or chiefly British with to<a very different situation to the ... one under which we live -- Sir Winston Churchill>

I am sure you want to say usage among highly educated groups and the press, whatever their level of education, drives the rule. If we let the rabble make the rules, we will all be saying "I could care less," which is nonsensical. We have, however, learned to embrace "Have your cake and eat it, too," which is nonsensical but has completely replaced the original and sensible "Eat your cake and still have it."

When usage is common but you don't like it, you are likely to find many whose beliefs differ from yours. Even most news reporters in America now say or write different than as a matter of course. It seems to be the new trend in American English in much the same way that as well has recently begun to replace almost completely our venerable American choice of also. Oxford English Dictionary (http://www.askoxford.com/concise_oed/different?view=uk ) seems to allow all three phrases but fails to make any nationality claim for different to. Perhaps they (or is a dictionary staff singular?) are not aware that Americans almost never use different to. Are we to say the British are wrong? Isn't it their language? I myself feel most comfortable with different from except in some odd usage such as "My hair color is different from that of Tom. Your hair color is different from that of Tom, but mine is more different than yours."

Two other small details. You should be careful not to split your infinitives, as you did with "is to not be the same as it". For better style, avoid using however at the beginning of a clause or sentence when the meaning is nevertheless. You may, however, use it at the beginning of a sentence or clause if your meaning is in whatever way or to whatever extent. At least that is the opinion of the late William Strunk (http://www.bartleby.com/141/strunk3.html ).
 Marius Hancu's reply was promoted to an answer.