Sharp Sidekick LX 2009
According to phonearena.com...
If that is true, then it is compatible for on AT&T's 3G network.
http://www.phonearena.com/htmls/T-Mobile-Sidekick-LX ... »
Early prototype versions did, and an early draft of T-Mobile's spec sheet did say that, but the final version does not have it, and the final T-Mobile spec sheet shows that. I've confirmed this with both T-Mobile and Sharp.
The FCC approval does not include WCDMA 850, so even if you could somehow enable it, it would not be legal to use.
Furthermore, you would also need WCDMA 1900 for full AT&T 3G coverage. You'll only get partial coverage (spotty in some cities and none on others) with just WCDMA 850.