Lately Apple has been incrementing the version number (and plugin compatibility UUID) of Mail.app with every version of 10.6. This breaks bundles like GrowlMail even when they are still compatible. The easy fix, assuming that the bundle is indeed still compatible, is to simply add the UUIDs to GrowlMail.
The attached app does this by running a simple shell script to get the current UUIDs and perform a little housekeeping with them. It will even re-enable the bundle if Mail.app has helpfully disabled it for you.
Simply download the app, unzip and run it.