We don’t have to tell you that in Safari you can enable Show Develop menu in menu bar (Prefs/advanced) then go to Develop/Useragent iPhone to get free wifi on your MacBook do we?
Update: The fine folks in the comments have pointed out that yes, you will need to have an AT&T iPhone account to do this. Is there anyone left who doesn’t?
Update 2: OK, the easiest way to do this is to download a MAC address spoofer and once you legitimize your iPhone, change your MacBook Airport MAC address to be the same as your iPhones. This is much quicker and easier than typing the URL sent by AT&T.