I actually have to agree with AT&T on this one, even though I hate those greedy thieves! Their 3G speeds in California are giving me an average of 2-4Mbps on my 3GS, which is good enough for me. I would obviously like a faster speed, but I can't imagine anything slower. Considering Sprint's max is at 2Mbps according to its iPhone website, I might not be as eager to switch to Sprint, until an iPhone is compatible with whatever their version of faster data speed network is...
I'm in California and actually see slower speeds on AT&T versus Verizon. EDIT: (Typically, in the areas I frequent).