If the BlackBerry Bridge application sounded like a nice option to let you work on your larger PlayBook, then you're out of luck if you're on AT&T. AT&T has blocked the BlackBerry Bridge App.
BlackBerry® Bridge™ mirrors the existing email, PIM and browser apps that are running on your BlackBerry® Smartphone, allowing you to view those apps on the BlackBerry® PlayBook's™ larger display
BlackBerry Bridge isn't really tethering. It will allow you to get online on your PlayBook through your BlackBerry smartphone so that you can access your email and browse the web, but that's because it's mirroring the apps on your smartphone.