There are essentially two ways you can do it.

Option 1: You need to know someone with an AT&T Wireless account. Simply use their phone number to receive the SMS with the coupon code. Go to www.apple.com and follow the simple process. They do not have to be an iPhone users, but it seems that they need to be an AT&T Wireless (or Cingular) customer.

Option 2: Get a pre-paid AT&T Wireless card and use the number on it to receive the SMS during the rebate process.

I personally was lucky enough to have US colleagues with AT&T Wireless accounts. I obtained the rebate certificate for both of my iPhones using two different AT&T Wireless numbers.

I have an at&t number if anyone wants to go 50/50 on the rebate. Just PM me.