Thanks to TDobes note on the Drupal forums, I got past a fatal error after upgrading to Drupal 4.5.1 from 4.3.

"Fatal error: Class theme_example: Cannot inherit from undefined class basetheme..." 

It turns out I had not read the instructions:

You need to remove ALL the old drupal files, then unpack the new files into the drupal directory.

Then it works, magically!