I'm overseas, so I've contacted HTC support by email, and this is their answer:
I understand about receiving the Android 2.3 update not being in an AT&T service area. I know how important it is to have your device up to date. I will be happy to assist you. As long as your device is activated in some fashion you will be able to receive the update. When the update is ready for your device, you will receive a notification alerting the Gingerbread update is ready for install. Once the update is released it may take up to 5 weeks for you to receive the notification. You may also manually check for updates by going to Settings> System Updates> HTC Software Update> Check Now. If there is an update available, you will be able to follow the prompts to start the update process.
I'm not sure what they meant by activated in some fashion tho.
Anyways, keeping an eye on HTC downloads page.