I am certain the answer to the original question is "Yes", you can port in to AT&T on a pre-paid or post-paid plan. AT&T Wireless offers voice-only monthly contract plans. AT&T also offers decent pre-paid plans: $50/month with unlimited voice/text (I think the base contract plan is $39.95 for 450 min.), or $2/day unlimited voice/text only on the days you use the service.
You can activate an AT&T pre-paid SIM either in a non-iPhone (you must be in the U.S.), or on-line using an AT&T iPhone IMEI. You can then use the SIM in an unlocked iPhone. You can add a data plan on top of a pre-paid SIM. I have colleagues who have done this many times.
The biggest challenge is cutting a normal-sized SIM down to a uSIM.
In any/all cases you should not tell AT&T you have an iPhone.
edited to add: You could also port in to Straight Talk, the pre-paid MVNO using the unlocked GSM SIM (uses AT&T's network) ($14.99) and a $30 or $45/month plan.
Last edited by transpac; Jun 2, 2012 at 10:24 pm