Using this simple terminal hack you can enable growl mail for leopard mail.
Here's what I did:
1) quit mail.app
2) reinstall Growl Mail
3) Run this in the terminal:
defaults write com.apple.mail EnableBundles -bool NO
As for me, I didnt need to set BundleCompatibilityVersion because it already has been set to integer 3 on a freshly installed Leopard. thschunde, I reckon that theres no need to re-set this to a different value.
I don't know if I'm being particularly dense, but I simply can't get this to work. As in other comments it's there in the prefs, everything supposedly is enabled, I've typed, and copied this hack and still I'm getting nowhere. I either get "unexpected argument" and nothing works, or, it has apparently run correctly, and still notification doesn't work. I'm perfectly prepared to believe it's my error, I just haven't got a clue what I'm doing wrong.
desperately clinging on to the last few shreds of sanity
I can see it in the prefs, too. But it isn't willing to show any notification. So I think, we have to wait until there's an official new GrowlMail out.