I am not running the Atrix on T-Mobile; I happen to have another phone (Nokia N900 on T-Mo) and wanted to be able to check my bills from the Atrix, same as I do my AT&T bills.
Since you are carrier unlocked, you should enjoy the same speeds as you get on AT&T. T-Mo's 3.5G is essentially the "4G" you get from AT&T on the Atrix.
There are a couple of TV apps on the Market -- SPB TV has been around for awhile. If you have Xfinity TV at home, there's an app for that. Search for TV in the market.
Are you having any issues downloading apps from the Market using the T-Mobile SIM?