While I didn’t have any issues upgrading other WordPress sites, one of them proved pretty tricky. It seems as though a fefw folks were having similar problems logging into their blogs following the update and a clearing of the cache seemed to do the trick. It did not for me. My problem actually lay in the plugins (which one(s) I’m not exactly sure) but here is my solution to getting around this particular login problem:
1. Copy all plugins from your server to your computer (they’re in the wp-contents folder)
2. Delete all plugins from your server
3. Log into your blog using either /wp-admin or /wp-login.php
4. Update the database
5. Log in to your blog again
6. Put plugins back on the server one at a time
7. Activate or upgrade each one if offered
8. Blog on

It took me a while to figure this out (with the help of others) so I thought I would share my experience in case anyone else is freaking out about being locked out of their blog.