This occurred in 3.2 as well and I could never get it resolved but had narrowed it down to a "bad" theme that I installed. I removed that theme but could never get that page to load again so I cannot network activate any new themes, etc.
I upgraded to 3.3 today and upgraded the database and the page loaded once again. Yay!! However, when I updated one of my themes, I get the same error.
Now, I believe I know which theme is the problem and when I downgrade or remove the theme entirely from the directory, the theme.php page remains white - which I belive is a PHP problem but that page should not be really changed, I wouldn't think.
My best guess is that when I upgraded the Database, something was changed and fixed but now I cannot run the upgrade again (or can I?). If I hit upgrade.php, I am informed that my DB is up to date.
Thanks in advance.