Following up, users of venerable forum XDA-Developers have confirmed that AT&T has disabled the capability to install Android applications from sources other than the default Android Marketplace on its build of Android 1.5 via the device.

This means that attempts to load applications from the SD card, over Wi-Fi or unsigned applications will not work on the device in its stock form and requires the setup of the Android SDK with drivers in order to setup ADB access via the USB Debug Menu as a workaround.