Apparently AT&T is struggling a bit with the whole idea of Android, a somewhat open mobile OS. Instead of embracing it and giving users a full experience, they've decided to cripple it and not allow the installation of non-market apps.
From the sounds of it, the Android OS allows for the installation of apps "purchased on alternative markets and beta apps like Swype" by default. It's a bit of a mystery why AT&T would choose to take this option away from users, but it certainly makes AT&T's first Android-based phone even more of a letdown. [XDA Developers via Android and Me via Engadget]