Thanks so much for starting this thread, especially to Stephan Blake for posting the step by step instructions!
I would also like to say that if anyone has to contact Straight Talk with assistance activating the ATT sim card that comes with the start up kit, tell them that you are activating an ATT phone, it will save you a lot of headache. I told the gentleman on the phone that I was activating a verizon Samsong Galaxy SIII, and that I could do it using the SIM (after reading this post), and he insisted that I would have to do it using the CDMA format. I told him that it would not work that way, he insisted that it would! So I said "Ok, lets do it using the CDMA method!". After he was all done setting it up, he told me that it would take an hour for it to finalize, and told me reboot after an hour, and input a code. I said "Ok, I'll be calling you again tomorrow when this don't work". The following day I called them again, because it didn't work. But this time the lady on the phone asked me if I was activating an ATT phone...... I said...."Yes", and it went so much smoother, was done and off the phone in about 20 minutes with the ATT sim card activated, and then I followed instructions that Stephan Blake posted..... Works like a charm!
So again, if you need to call Straight Talk for any reason during setup, just say "I'm activating an ATT phone"... =)
Good Luck! =)