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That patch works but there is a procedure to follow after you install it. Check the GrowlMail forums to find it. Glad they fixed it because GrowlMail is the biggest reason I paid for growl.
There's a procedure? I just downloaded it, closed Mail, ran the installer, and that was it - nothing special to be done, and it seems to be working fine. I see that some people did have issues, and it seems that their fix was to remove the old version first and then install the new one.
Strangely, while it was working fine for me yesterday, today it's not working. Based on discussions from the Google group, it seems that the developer is looking into it and hopes to have an update to fully fix it soon.