It would depend on the radio it has inside it.
Since the Lumia 900 is going to AT&T right away, the phone will only support WCDMA Band II 1900 (it also states that on the Nokia US website) for data. T-Mobile requires Band IV 1700. So, while the phone will work just fine on T-Mobile, it will be slow as molasses for anything data related.
None of that is going to matter once T-Mobile refarms its 1900 mhz spectrum for data usage but I doubt that'll happen this year and I doubt it'll affect EVERYBODY at once anyway.
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Replying to the original post, it seems like there is the possibility of the 900 coming to T-Mobile this year, after AT&T's exclusivity window ends, but there's nothing definitive as of yet (Who knows, maybe the exclusivity window is much longer than 45 days).
My solution to this is to buy an unlocked Lumia 710 in the next month and just wait until either the 900 if it launches later this year, or an Apollo device. I still have a phone upgrade on T-Mobile so I'd rather use it on an expensive device. Being that I'm currently on an Android phone, the above is the logical course of action for me. I'm tired of performance issues that have plagued me on the Nexus S as well as lower end Android phones.