Ok, I'm not exactly sure why this worked, but this is what just happened...
1. Searched Google, came up with a few posts regarding the update to 5.3, and sure enough: some units got bricked. One of the commentators suggested that instead of updating it via an ethernet cable, to use wireless instead.
2. I had originally tried to update the ATV via a wireless connection, and that's when the **** went down; I tried resetting it via removing the power cord, and when that didn't work, connected it to iTunes to reformat it. Neither worked.
3. Called Apple Support. No help.
4. Decided to try (nothing to lose, right?) hooking the unit to the router via an ethernet cable, and....everything woked.
5. Went to Settings, checked that it was indeed updated to 6.0, and all my passwords, etc., were there.
6. So, moral of the story is: if you've already reformatted it, use a direct connection if you originally tried via wireless, or wireless if you originally tried via ethernet. Why is mine now working? I haven't a clue. Good luck!