I haven't noticed anyone posting this yet, but I have seen the question asked numerous times. According to Think Secret and a sources' test drive - AT&T SIM cards ARE interchangeable in the iPhone. We should be able to use the included SIM in a "cheap" phone, or you should be able to use your existing SIM.
http://www.thinksecret.com/news/0706iPhonehandson.html
http://www.thinksecret.com/news/0706iPhonehandson.html