applications - How to install from unknown sources on AT&T?

A friend of mine has an HTC Aria on AT&T. On his application settings screen, he does not have the option to allow installation of apps from unknown sources. Is there any way to change this option to allow him to install downloaded apps (*.apk files) without rooting the phone?

Answer

Unfortunately, it seems like the HTC Aria does have that ability, but AT&T has removed it.