got/gotten

I remember reading once that there was a difference between got/gotten.

Most grammar books say that they are the same.  Does anyone know the difference?

Thanks Calif Jim.  That's what I was looking for.  I thought there was some kind of rule for "have got" as opposed to "have gotten"

Inever thought of it as being an idiom.  Thanks again.

I found this on the Internet and the explanation is similar to what Calif has explained.

http://www-personal.umich.edu/~jlawler/aue/gotten.html

Here's what David Crystal says about The gotten/got distinction in
The Cambridge Encyclopedia of the English Language (p.311):

"Gotten is probably the most distinctive of all the AmE/BrE grammatical
 differences, but British people who try to use it often get it wrong.
 It is not simply an alternative for have got.  Gotten is used in such
 contexts as 
    They've gotten a new boat.   (= obtain)
    They've gotten interested.   (= become)
    He's gotten off the chair.   (= moved)
 But it is not used in the sense of possession (= have).  AmE does not
 allow
   *I've gotten the answer.
or *I've gotten plenty.
 but uses I've got as in informal BrE.  The availability of gotten
 does however mean that AmE can make such distinctions as the following:
    They've got to leave  (they must leave) vs
    They've gotten to leave  (they've managed to leave)."