- run Mail, let it disable the plugin
- quit Mail
- rename the Bundles-disabled back to Bundles
- Open Terminal
defaults write com.apple.mail EnableBundles 1
- restart Mail
The plugin should work again. Note that we're using 1 and not YES (like the plugin docs for 10.4.0 said).
According to the mailing list, if you follow the previous directions from the dmg, and set BundleCompatibilityVersion to 1, that triggers the new Mail in 10.4.1 to complain and disable the bundle.