I replied to another post about this very thing, but I want to make sure it's easy to find as well...
I just got this error when trying to upgrade via Dashboard (hosted by MediaTemple):
Fatal error: Cannot redeclare class Plugin_Upgrader in .../wp-admin/includes/class-wp-upgrader.php on line 632
Is there some way to get around this to get the installation to work? A note: I am using the default theme to which I added a little CSS.