Which phones allow this? I'm honestly curious, I thought AT&T had switched that off on all models. Is yours grandfathered in or are they releasing new models with unknown sources enabled?

I believe even the AT&T phones with Unknown Sources disabled allow sideloading with ADB. Not that that's a good option for most users.

Couldn't reply threaded.

@jancona: I didn't sideload with ADB. I copied the .apk to my phone via USB and used a file manager to access my file system. Tapping on the apk installed it just fine. If AT&T disables sideloading with ADB, how do developers test their apps on AT&T's Android offering?

@lukeschlather: I'm using a Nexus One; it may not be possible for phones customized specifically by AT&T. If this is the case, it's pretty f'n unfortunate.