The $20 unlimited internet for my GoPhone works great on my 3G iPhone...
But yes, they do monitor it, and see the IMEI number, and sent me a text message they other day...
See my other thread:
AT&T Sent Threatening Text Message
The only true way around this, is to change the iPhone IMEI number to the IMEI number of you flip phone that you want them to think u are using.
People on eBay probably bought them and wiggled out of the plan and early termination fee, or purchased it outright on release day when the activation servers went down... --That's how I got mine... Told them my Dad wanted to upgrade to the 3G iPhone, paid $200 for it, then put the new SIM in my Dad's old phone, called AT&T and told them my Dad was not ready for the switch to the new iPhone yet, and we'd call back when he was ready, and they put everything back to like it was... So then I just put a GoPhone SIM in my new iPhone. :-)