Bridging is supposedly legal. You are basically downloading data to the phone and syncing the data.

ATT is going after iPhone users right now who tether. They don't seem to be going after Android or Blackberry. My guess is that they are packet sniffing and don't have the confidence to call a "cheat" on these users. PDANet, by the way, has just upgraded their iPhone net to hide their tether application from ATT. It looks like it dynamically modifies the packets to make them appear as if they were from an iPhone.