So you're using a custom rom or you're rooted. I thought it was obvious I was referring to stock. You can also run whatever you want on an iPhone if you root.
@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.
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?
Given his "Stock 2.2" comment, best guess would be the Nexus One with AT&T bands. Since it wasn't sold by AT&T, it doesn't have the APK installation locked down.
On, uh, AT&T.