If Nokia is going the same path as the E71, they will release the N900 unlocked first, and maybe 6 months down the road announce a subsidized one with T-mobile. (The E71 was widely available in the fall of 2008 and didn't come to AT&T until spring of 2009) In the process of that 6 months, I'm sure they're going to use that time to beta-test on those here, who are rushing out to buy the unlocked version, to shake out the bugs before they hand over a less buggy version on the T-mobile branded version of the N900.

I bought one of the unlocked E71 that was truly buggy since it basically melted itself 4 months after owning it.