T-Mobile is going to have 4G before AT&T(Although they aren't calling it 4G they are calling it HSPA+, the speeds meet or exceed 4G specifications). This same technology will improve speeds on most current 3G T-Mobile compatible devices as well.
http://reviews.cnet.com/8301-12261_7-20001045-10356022.html
HSPA is 3G technology. TMo wants to call it 4G because it'll probably be in line with 4G speeds on VZW and Sprint. The fact is W-CDMA has a long way to go and you can go to 7.2mbps, 14.4mbps and even 21mbps, and beyond before AT&T really has to switch.
Like I said before Sprint and Verizon have no choice because EVDO-A is the end of the road. It's worse than AT&T's 3.6mbps network in terms of speed. That's why as AT&T deploys 7.2mbps, Sprint/Verizon HAS to either go to EVDO-B (which Verizon publicly said it would not) or move to 4G. That's why they have to move NOW.
T-Mo and AT&T have it easy, and while 4G is nice, deployment isnt imminent or isnt necessary YET. The rest of the world feels this too. This whole last year has been a move from 3.6mbps to 7.2mbps and beyond for many Asian and European countries. Many of them will probably even move to 14.4mbps too or 21mbps. The fact is 3G isn't dead yet, and this is shown by the # of phones still being released.
There will be 4G devices released and it seems like they're mostly to appease Verizon and Sprint, but don't expect wide releases of phones till 2011. It's just like AT&T had a 3G network from the get go. No one really cared about phones like Nokia's N-series which had 3G all along. No one cared about 3G period till the iPhone said here's a 3G phone. OOoooooh went the crowd. 3G!!!! Then all the data hogs jumped on board, and all the rave about tethering started. That's when we truly needed 3G. And right now we don't need 4G just yet even if VZW will deploy it first.
People laugh at AT&T for not deploying 4G fast enough, but the fact is the rest of the world is committed to using 3G till the last drop. And last I checked the US doesn't exactly dictate the phone market either as most phones are released for Europe/Asia with US ports. The exceptions have been the iPhone and the Droid, but it's not like they were releases solely for the US only as they soon circulated worldwide. So till 4G becomes solid, I doubt device manufacturers are going to jump on board and start building flagship phones based on it.