Using the mailbundle template found on github, I created this bundle from scratch for OS X Lion.
It simply adds a toolbar button and menu item to the Mail.app toolbar that, when clicked will add your email to a case in SalesForce.com. If the email already has a SalesForce.com reference code in it, it will automatically locate the case. If not, it will prompt you to search for the case.
I host the binaries in my personal dropbox and the app uses sparkle to keep updated. This is critical as each OS X release has new compatilibity ids that must be added to the bundle's info.plist. See package.sh for the inner workings.
Also I've included the proper Xcode config to allow debugging of the bundle while Mail.app is running, very handy.