I don't have ATT and have no insight there, but my suggestion is for you to get the BB and try it with the iPhone data plan first, although you won't have all features, it will give you a good feel for the device and it's functions before doing the data switch. That is unless you already know for sure that you will not want to go back to the iphone. Hopefully someone that has ATT will chime in here with advice on this.

Just my 2 Cents worth.