Give this a read: On iPhone, beware of that AT&T Wi-Fi hot spot - CNET
Looks like if you have attwifi saved as a known network, then yeah the iphone could connect to any of them. You have to be within range of one in order to select the option "Forget this network"
Or, you could just reset all of your network settings. Go to Settings > General > Reset > Reset Network Settings. This should wipe out all remembered networks but could also erase some important settings.
Got me thinking.. maybe I should setup an attwifi network and see how many poor saps connect to it.