But most of us have been saying ‘two thousand and X’ for the last decade, so why should we change style now?
A Times leader has half of the answer:
At present euphony dictates “two thousand and ten”, on the model of how the film 2001, A Space Odyssey is promounced.
Yes, “promounced”. Oh well. But the other mistake there is to forget that we also have a very well established precedent that supports ‘twenty-ten’. It’s one of the best-known years in all of English history.
Don’t try to tell me that you read that as ‘one thousand and sixty-six’. So it seems fair to conclude that once we get past the first decade of a millennium, we go from ‘two thousand and nine’ to ‘twenty-ten’.
On the subject of what to call the decade, that’s going to be harder (the 2010s works - no apostrophe please! - but we seem to need an abbreviation). I think the Times’s suggestion of the ‘Tennies’ is the best I’ve heard.