I've never seen anything "official" for a launch date, although T-Mobile users in DC and Atlanta (?) are reporting EDGE is active on certain towers. General guess is "later this year".
EDGE will bring T-Mobile up to par with what the other carriers have had for the last year or two. While T-Mobile is focusing on EDGE, the other carriers are all moving onto the NEXT thing (EV-Dx for Verizon/Sprint, and UMTS for Cingular).
If data speed is very important to you, I wouldn't recommend T-Mobile.
OTOH, BlackBerrys (for the most part) mask the speed of the network that they use. When we had T-Mobile Pocket PCs, we got complaints from our users all the time about how slow the network was to replicate their email. Now that they all have T-Mobile BlackBerrys, we haven't heard a peek from them -- their email is simply "there". No more clicking on "replicate" and waiting 2-3 minutes.