There actually might be something working against the "twenty twelve" pronunciation: the Mayan prophecy.
The year 2012 is now so closely linked with the Mayan calendar, and "twenty twelve" doesn't sound very ancient and/or prophetic. "Two thousand twelve" has the weight of history.
Don't believe me? The movie "2012" used the "two thousand twelve" nomenclature...in the trailer, at least. (I didn't actually see the movie, which came out in 2009. Hard to believe it was that long ago.)
Of course, the same trailer also calls the Mayans the "world's oldest civilization." I guess we're not counting Mesopotamia, Egypt or China.