More to add to previous post:
1) I went to the ATT store today and asked if they object to prepaid SIM use with a iPhone. I was told that they do not object, there is no policy against it - but if you chew up minutes on lots of data, they won't help. So, keep 'Cellular Data' OFF in Network settings of your phone.
2) The store clerks told me that they could not port my existing cell number to the SIM. They were wrong. I called Customer Service, told them I wanted to port the number and they connected me with the Porting Department and it was done in a few minutes.
3) Having a Google Voice number is a great addition to your iPhone. With the iPhone GoogleVoice and Talkatone Apps, you can avoid using minutes for calls and texts whenever you have WiFi. GV has lots of other neat features like a single Voicemail box for both cell and GV numbers, email notification of VM, transcription of VM, etc. Access to VM is then free whenever I have WiFi or am at my computer.
4) When logged into my ATT account, they 'know' I have an iPhone, it shows up on the support page. Another indication that they don't care if you are using a prepaid SIM in an iPhone.
Bottom line, if you can live with WiFi data only and don't make a lot of calls or texts, a prepaid SIM works just great. If you need lots of calls or texts, there are prepaid options for that.