Okay, so I'm upgrading my Drupal site and get the "White Screen of Death" when I run update.php. Of course I start sweating thinking "Oh my Gaw! My site can't be updated and I won't be able to add content and people won't see it and what's going on!"

Just for the record, last time I added content to my site was...
So you see why I'm in a panic.

Anyway, after lots of emails between myself and my web host's tech support I still had a white screen, but I knew that my site was running PHP5 and how to use phpinfo.php to check that, although in my control panel PHP4 was still showing.