- Obtain an AT&T compatible iphone (new or used). I don't think you can buy one at Walmart without signing up with AT&T or Verizon, but many other places cell them contract free.
- Purchase a $15 AT&T compatible micro-sim card from Straight Talk and insert it into iphone.
- Activate the micro-sim with Straight Talk (if you had a phone number from another carrier that you previously were using with iphone or any other cell phone, at this stage you can assign it to the micro-sim). It may take up to two days from the time that you activate and the time that your iphone can begin to place and receive calls and/or text messages. At this stage you pay your $45.
- Set the APN numbers on the iphone to work with Straight Talk (this step is necessary to use data services). One way to do this is to connect iphone to wifi and then go to go to http://www.unlockit.co.nz, and the iphone will automatically adopt the proper APN numbers. Update Oct-2013: follow these instructions to get your iphone to automatically adopt the proper APN numbers.
Update Oct-2013: I did not correctly predict AT&T's pricing. It appears that AT&T is actually charging a bit more (eg., no more 200-MMS packages) than they used to.
Update Jan-2014: AT&T has finally moved prices a lot closer to Walmart.