iClarified has posted the following:
How to Capture Your iPhone 3GS iBEC and iBSS (Mac)
How can I properly perform this if I have a 3GS with only T-Mobile service at the moment... I can put the phone into DFU mode and get that portion of the steps done (iBSS) but not the beginning part (iBEC) as my phone is unactivated... any thoughts? Can I use someone else's AT&T SIM to perform this?
Thanks for the clarification. I just want to be prepared for the JB and then the Unlock.