Pay-as-you-Go Locked-iPhone4 on H2O
I will apologize in advance if this is a well worn subject. I just set up my friends ATT-locked iPhone4 on H2O Wireless (formerly O2?) and it worked perfectly. No jail-breaking, no unlocking, just simply cutting down the SIM card and activating per H2O instructions. I believe there is an issue with MMS messages but my friend didn't care about that.
He bought his phone used on eBAy. We threw away the ATT micro-SIM and replaced it with the cut-down H2O SIM. He purchased the $60 Unlimited Talk/Text with 2 gigs of data (I would only recommend this for people who are usually on wifi).
Again, I am an Android guy, so I apologize if this is old news.