T-Mobile will be following through with their addition of the 1900MHz frequency bands to their network. They'll be updating their towers from here on out, and this will make many current T-Mobile customers extremely happy; the 1900MHz bands should allow unlocked and imported devices to finally reach 3G speeds.
Sure Edge speeds let you access the internet, but it's those with an unlocked speedy smartphone on T-Mobile's network should at least be able to access 3G signal. There's no idea where they'll rollout the updates to the network first, so unlocked iPhone users just keep an eye on your device to see if you finally connect to a 3G network. Maybe we can find an update pattern.
Most future devices from T-Mobile will probably come equipped with capabilities for the new bands - but until then unlocked/international devices will be the only ones compatible. Personally, I was expecting an update to 1900MHz from T-Mobile due to their suggested AT&T buyout deal. Since that has fallen through, hopefully they continue with their plans.