OTA Updates Coming to Allow Sideloading on Older AT&T Android Phones
Sideloading is a term that means the ability to install builds from non-Android Market sources. That could be another store like the Amazon Appstore, or it could be an APK file on an SD card. To enable it, you need to go into the device settings, and allow "Unknown Sources" to install.
The updates that are coming cover the HTC Aria, the Samsung Captivate, and the HTC Inspire 4G. The recently released Samsung Infuse 4G came with sideloading enabled. Based on AT&T's informational page on the matter, at the very least the Motorola Atrix 4G will get the fix, as well.
The updates are coming OTA, which means it's hard to predict when they will arrive, but they are going out now.