I was reading the usual tech sites and came across
over at TUAW. It was about using an O2 SIM card (not the U.K. phone company, the U.S.A.
one - which is now called H2O) in the iPhone 4. All you had to do was activate the SIM,
then cut it down to fit into the iPhone's micro SIM card tray and it just worked. Of
course these SIMs will all work in iPhone 5 as well (provided you can unlock it or you
have a factory unlocked model. I hear the AT&T and Verizon models are unlocked.).
Perhaps you received an iPhone as a gift and don't want to sign a two year contract with AT&T, or maybe you just aren't much of a talker but still like smartphones, or maybe you don't live in an area serviced by AT&T. Whatever your situation is, you have other options. You can get data using an O2 SIM (now called H2O, they resell AT&T service). However the service plan requires a voice plan and comes with limited data.
If you have an unlocked iPhone then you can use Simple Mobile or T-Mobile. You can get data with both of these companies as well. Simple Mobile resells access to T-Mobile's network so you will not receive 3G service using an iPhone. I purchased a Simple Mobile SIM from their website here. They also have a network of resellers in various retail stores so check their site. You can get data, tether, use MMS and FaceTime on Simple Mobile and T-Mobile. Simple Mobile will require you to pay every month: you can't keep your minutes over a long period of time like you can with H2O or T-Mobile pre-paid accounts.
It is possible to restore the iPhone with an inactive AT&T, Cingular, GoPhone, H2O, and a newly purchased O2 SIM. they will activate the iPhone. You can purchase airtime recharge cards at most convenience or electronics store. You must purchase an AT&T GoPhone SIM directly from an AT&T store so that it will work with your iPhone. If you purchase one bundled with a phone it most likely will not work.
I purchased an O2 pre-paid SIM card at Best Buy. You can see their site here.
The next step is to cut your SIM down to the size that the iPhone 5 will accept. I used the Noosy SIM cutter which I purchased here. The cool thing about this is mine came with SIM adapter trays to convert the nano SIM back to a micro and a full size SIM.
There is a Wiki that lists (based on country) which pay as you go SIMs are available for your iPhone.