Replying to AT&T vs Me: Who wins?

Set up:
Bought my new iphone 3G S last friday.  Got a call from the AT&T store that I bought it from saying they didn't have any receipt in their records of me purchasing it. They want me to come by with my copy of the receipt or come by to pay them for it if I can't find it.

Here's the thing, I don't remember the lady asking for my credit card, or payment or anything, and because I have my account set up to auto pay, I figured she just used that...

I haven't looked for the receipt yet (still at work).

Question for you:
If I can't find the receipt, and I actually DID just walk out of there with the phone for free after they set it up and everything for me...can I just ignore them and not get in trouble and get a phone for free?

It's probably a dumb question, but I figured I'd ask.

Any lawyers in the house?

Thanks ahead of time if you can help me out.