A Psychology professor at UC Berkeley said that dah-ta (like "dad") is the singular, while day-ta (like "today") is the plural. In other words, you would say "these day-ta", but "this dah-ta."
I haven't heard this from any other source, and I've also heard another Psychology professor at UC Berkeley say "day-ta" for singular. When I've worked with people who process data on a regular basis (in Psychology research labs), we almost always use the plural form, so I haven't had much chance to see how someone would pronounce the singular form.
Honestly, this is some petty stuff that doesn't matter, though it's one of those things that makes you curious because you want to know what is "correct". I have the same feelings toward the who/whom controversy--it doesn't matter enough for me to know how to use it, but I still want to know which is "correct".
I think the only way to resolve this issue is to hit up the Oxford English Dictionary or a professor of English who really knows the nitty gritty.