Feedback from my recent trip in the US with my Vodacom iPhone 4S. I got the AT&T GoPhone pre-paid bundle and loaded about 250 MB on my phone - used it without any issues!
Went to the first AT&T store I could find, but took along an old Nokia E65. Told the guy I wanted to buy the GoPhone SIM WITHOUT any Go-phones - since I already have one. Apparently some of the AT&T stores are rather useless and you have to really convince them that you don't want to use their phone. Don't tell them you have an iPhone/Android - they usually tell you the phone isn't supported on prepaid.
There are three pre-paid plans: the one is day to day which costs $2 per day to activate. You then get unlimited calling and US-texting on the day. This does NOT work with the data bundles - you have to take the month option (which costs $25 and gives you 250 mins for the month and 1000 US-only texts). You can then request data bundles (make sure you you don't buy the all in one bundle package which gives you 'unlimited' data and calling etc, but rather the separate 50MB, 200MB or 1GB bundles). They are $5, $15 and $20 if memory serves correctly.
I initially took the 200MB bundle to test it out with, but could have taken the 1GB from the start!
Got where I was staying and tested the sim on my Nokia first. Worked without a hitch. Then proceeded to cut down the SIM for my iPhone. Also, make sure you put your iPhone on a Wifi connection first, as in the last step you want to set up a new APN using
www.unlockit.co.nz on your phone's web browser.
You can register the SIM online at
https://www.paygonline.com/websc/index.jsp and do online refills for data & airtime with your SA credit card.
Interestingly enough, my iPhone started showing "4G" as soon as the data and everything was activated - pretty lame (
http://abcnews.go.com/blogs/technology/2012/03/att-iphone-4s-update-has-4g-kinda-sorta/).
Last thing that I found useful for calling the wife is the Telkom World Calling cards. Bought a R50 one and took the pin along. The number in the US to phone is on AT&T, so you can use the 250 Go-phone minutes to call the Telkom number. I then proceeded to call her on her Vodacom / CellC number in SA and it used very little funds - spoke a lot on the calling card and didn't even use up the R50.