T-Mobile 3G in San Francisco is fairly good. I tether with it all the time, but T-Mobile uses a different frequency for their 3G so your AT&T phone can hop onto

T-Mobile 3G in San Francisco is fairly good. I tether with it all the time, but T-Mobile uses a different frequency for their 3G so your AT&T phone can hop onto T-Mobile Edge network but not 3G, unfortunately. Same thing in the other direction. I don't think any manufacturers make phones that can do both frequencies for 3G.