Mac OS X 10.6.4 is out and with it, Mail 4.3 which breaks GrowlMail, Letterbox, Letter Opener and DockStar. All but DockStar fixed though.
To fix Growl
Download a pre-patched GrowlMail.bundle and drop it in your ~/Library/Mail/Bundles/ directory. If you want it available to multiple users on your system, use /Library/Mail/Bundles/
To fix Letter Opener
Close the mail application. In finder go to Macintosh HD /Library/Mail/Bundles (Disabled)
Copy OMiC.mailbundle to Bundles
Replace the old info.plist (located inside contents folder) for this one: Info.plist