3G can theoretically go up to 7.2mb/s depending on the area. However realistically you can exepct approximately 1 mb/sec 3 if your lucky.
As for using AT&T's netowkr to replace your home internet, that's a violation of AT&T's ToS if your not using a plan where your allowed to tether. Replacing your home internet with an AT&T Data plan that's not approved for tethering will raise red flags on your account. There have been cases where customers who were not on approved plans for tethering being hit with PPU charges and having the accounts permanently suspended.
And as for the T-Mobile Question, the T-Mobile 3G network at this point is limited to major urban centers, and uses a totally different frequency compared to AT&T's 3G network, it is unclear whether the upcoming 3G iPhone will work on T-Mobiles 3G network.
And as for Blackberry's in certain scenarios, Blackberry's are very fast compared to other phones because the data is highly compressed.