Hello,   I just bought an iPhone 3GS from someone on craigslist so I can upgrade my iPhone 3G.  They said it was completely restored and that I could activate it by just putting my existing SIM in it.  I connected the 3GS to the computer with the SIM from my current 3G but iTunes said that the sim is not supported.  It states that maybe the carrier is different.  Its still an AT&T iPhone SIM and my account still has the iPhone plan.   What should I do so I can use the 3GS with my current iPhone account.   Question #2: Will the apps I purchased load on the 3GS?  Will my music load into the iPhone?  How does iTunes know that this different iPhone is valid to use with the apps and music I had in the 3G?   Thanks in advance!  I hope I get an answer soon as I would like to use the 3GS this weekend on vacation (especially the video part!!)