1. Close Mail
2. Move Growl back into the Bundle directory (if needed)
in ~/Library/Mail/
If GrowlMail.mailbundle was already moved to a Bundle (Disabled) folder move it back into Bundle.
3. Edit the Info.plist file and add the two UUIDs for the latest version
vim ~/Library/Mail/Bundles/GrowlMail.mailbundle/Contents/Info.plist
add the following lines around line 71:
<string>758F235A-2FD0-4660-9B52-102CD0EA897F</string>
<string>3335F782-01E2-4DF1-9E61-F81314124212</string>
4. Launch Mail and continue enjoying this awesome product.