To clarify for anyone who is still having any problems doing this.
1. Unlock your phone with the carrier ( I know the process will work if it is jail broken but I am assuming you dont want to void the warranty on your phone) NOTE: even if you purchased the phone outright at Wal-Mart it is still locked to ATT so you will have to unlock the phone.
1a Here is the link to unlock a ATT iphone. Simply follow the instructions https://www.att.com/deviceunlock/client/en_US/
2. Get a T- mobile SIM (micro SIM). Even if you don't want the service, if you feign interest, they will send you one for free or 99cents. If you need it now you can get one at a T-mobile store too.
3. Once you have both your Straight talk SIM and your T-mobile SIM follow these instructions:http://www.youtube.com/watch?v=mFFf5uqk18M
4. If you have any remaining questions check out the Howard Forums Wiki: http://wiki.howardforums.com/index.php/Straight_Talk_iPhone
Everything should work flawlessly at this point. I hope this makes doing this very easy for everyone in the future. God Bless -J