I don't believe you sign the contract when you order the system. You get the wireless card free and IF you sign the contract you get the mail in rebate. Even if you have no intention of getting AT&T service you can order it with the 3G card for free. The only reason I posted this is because some of the motherboards don't have the socket soldered on. This is a sure fire way to make sure your system has the socket ready to go. I had no plans of switching to AT&T (been with Sprint for 9 years) but I do want the socket for the 3G card to be there. And even if you don't want to use a 3G service, you can use the included card as a GPS receiver.
Also, they told me that if I wanted to add the card to my "in production" system, they would be glad to do it but I would have go with XP and the smaller hard drive. I didn't post this to complain about my experience, but to let others know that if they want to make sure they get a 3G slot that they can get the card for free if they can live with the smaller hard drive.