I think it depends on AT&T, but it really shouldn't matter. I know when I bought a phone from Amazon, al they wanted was a copy of my T-Mobile bill showing that I paid the $25 activation fee. I had a problem when I got my phone as well, it was pre-configured and they gave me an out of state phone number. I called T-Mobile and they changed it no problem. I didn't have any difficulties with my rebate. ALl they care about is that activation fee since thats how they are paid.
I was actuallu talking about Activation profits with my friend today whose dad owns a store. AT&T pays dealers around $300 for new activations. T-Mobile pays $270 per activation plus a $10 Gift Certificate anywhere. Pretty interesting...