Update (Oct 7, 2011)
The Drupal 7 upgrade process is notoriously buggy. And once you hit a bug, you have to restore your database from a backup and start all over again. I encountered a bug with "Table role_permission already exists" when upgrading from version 6 to version 7. This table is a leftover from a module that I'd previously uninstalled. The fix for it is to simply delete the table before you run the upgrade. Remember, once you see this error, you need to restore from backup and start the upgrade process again.