I have Version 5.1.0c (Build 4979)
I click Admin, Upgrade Wizard, and immediately get:

Fatal error: Class 'SugarThemeRegistry' not found in /var/www/sugar/modules/UpgradeWizard/uw_utils.php on line 3017

Per Kuske, this is apparently still an open bug, here:
http://www.sugarcrm.com/crm/support/...3-4a9553cb6734

This bug is two months old, I see. And it prevents people from upgrading. I'd think it would be a very high priority to get THAT fixed.

Anyway, I commented out the whole switch statement that line 3017 was part of, and that fixed it.