[Ken Case]

If you reinstall the Clip-O-Tron from OmniFocus 1.7.5, it automatically updates it for the current version of Mail on your current operating system. (This wasn't true in releases earlier than 1.7.5.)

I just verified that uninstalling and reinstalling the Clip-O-Tron using OmniFocus 1.7.5 on Mac OS X 10.6.2 makes it compatible with Mail for me.