Well, this is genuinely awesome: it seems AT&T is pulling back from its ban on third-party sideloaded apps on its Android devices, because HTC has just released an Aria update that enables them (either that or HTC just went rogue here, but we highly doubt it). While that doesn't have much practical implication for your typical smartphone user, it's a strong sign that AT&T might be ready to take a more reasonable stance on the openness of its branded devices that would put it inline with the attitudes Sprint, Verizon, and T-Mobile have all taken with their Android phones so far. Additionally, the update includes "the Mobile Network sharing function," which we take to mean a mobile hotspot app in the same vein as the one that's shipping with Froyo -- if we had to guess, it probably requires AT&T's 2GB DataPro plan plus the tethering add-on, which would put it in line with what they're charging on the iPhone. Onward and upward, AT&T.
Update: We got downright excited for a second there, but an HTC spokesman just called to inform us there's been a mistake -- this download is only for the desktop HTC Sync client and doesn't actually enable anything on the phone. We're working to get more information right now, but it sounds like the sideloading ball is still squarely in AT&T's court.
Update 2: HTC apologizes for the confusion, saying the update description was in error -- it was merely supposed to read that the Aria now works with the company's desktop sync client. On Monday, the text will be updated to remove every last ounce of false hope.
Update 3: Sideloading works after all! Read all about it.