I don't have the ability to uncheck the unknown sources box, in fact it does not even show up. I read something about a softwareupdate on this forum, but it just keeps giving me a server error. I just got the phone last week so I would assume it was up to date anyways.
Can I do 3rd party apps with this phone? I just tried doing the wifi update and it says no updates are available
There's a trick on the last page of that thread (Post 31), where if you change the date on your phone ahead a few weeks, it will apparently "trick" AT&T's system into pushing the update to you faster, rather than waiting for it to arrive.
If that doesn't work, you should be able to call AT&T customer service. Ask to be transferred to the department that can push out the update (to load 3rd party apps) to your individual device.