Hi Troy!
I had this exact same thing happen on one of my main websites.
Don't know if it's the same problem, but here's what I did to fix it:
- V3.5.2 seemed to update okay but I was kicked out of the dashboard right after it finished patching
- could not get back in
- noticed the 404 URL string was referencing "wp-admin/upgrade.php" could not be found
- copied in a new and fresh upgrade.php file
- Wordpress prompted that the DB was out of date and need patching
- DB patched -- everything working great again.
Don't know if this helps...but I can relate to the frustration. Let me know if this solves it for you.
Andrew