The 10.6.4 update broke GrowlMail by updating Apple Mail 4.3 UUIDs. A little web searching uncovered this page, and a little further searching in the Mail.app and GrowlMail .plist files uncovered the new numbers, and where to change them. If you can't wait for an official Growl update, you can get GrowlMail working again in a few minutes with the following steps.
Quit Mail and reinstall GrowlMail. This will make sure that GrowlMail is where it should be because when Mail 4.3 starts it moves GrowlMail to the 'disabled' bundles folder. You can move it manually but it's easy enough to reinstall. Here's the GrowlMail download link.
Run these two commands in Terminal:
defaults write ~/Library/Mail/Bundles/GrowlMail.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "B842F7D0-4D81-4DDF-A672-129CA5B32D57" defaults write ~/Library/Mail/Bundles/GrowlMail.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "E71BD599-351A-42C5-9B63-EA5C47F7CE8E"
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100618233944277