Great news! HTC‘s Syncing software now allows you to sideload apps onto HTC Phones on the AT&T network that are not rooted. Does this mean you can directly install applications from Amazon’s Market onto your phone. Unfortunately no, you will still have to root your phone in order to enable that functionality!
How to Root HTC ARIA!
UPDATE: Day late and a dollar short – HTC Sync has re-published their software without the software sideloading, if you have a version of HTC SYNC that supports this, do no upgrade it!