I think you are asing, since you cancled your gophone service with iPhone, and now want it reinstated with a new sim. The answer is NO. Everytime a new sim is inserted into an iPhone it automatically tells you to sync to iTunes for activation.To bypass this you need to jailbreak and or unlock. I did it myself, its pretty easy, and PAYG works pretty good on the iPhone. And AT&T told me it was not possible! (not possible my a$$!)
I upgraded to a 3g from v1. had a gophone sim in an old razr for a backup. put the gophone sim from the razr in the v1 and it works fine didn't have to reactivate or connect to iTunes etc. btw the sim from my 3g works in the v1 too w/out any problems.