Out of the general interest of consuming less time for our Developers so that we can get b3 (and hopefully not many more betas) out, I'm going to answer this once, and then let's wait until b3 is out.
If these instructions do not work for you, please wait for b3. It will have this change, and should be out shortly (no eta).
The GrowlMail.mailbundle includes a file called Info.plist. Inside this file is a tag that has the array with the 2 string lines in it already that look like this:
25 <key>SupportedPluginCompatibilityUUIDs</key>
27 <string>B3F3FC72-315D-4323-BE85-7AB76090224D</string>
28 <string>225E0A48-2CDB-44A6-8D99-A9BB8AF6BA04</string>
Now it needs to look like this:
25 <key>SupportedPluginCompatibilityUUIDs</key>
27 <string>B3F3FC72-315D-4323-BE85-7AB76090224D</string>
28 <string>225E0A48-2CDB-44A6-8D99-A9BB8AF6BA04</string>
29 <string>99BB3782-6C16-4C6F-B910-25ED1C1CB38B</string>
30 <string>2610F061-32C6-4C6B-B90A-7A3102F9B9C8</string>
The UUIDs for plugin compatibility changed in 10.6.1 from 10.6.0. So in other words, you have to have a mailbundle that matches the UUID in the SupportedPluginCompatibilityUUIDs. The link that Rudy provided (
http://growl.info/hg/growl-development/rev/4652f4f32776) shows the change explicitly.
So now with the updated UUID's, GrowlMail should work fine with Mail.
Chris