MailtoMutt now supports all the parameters I know you can pass to a mailto link - subject, and body. The communication with mutt is now done more correctly. Come download from the file list, and give us some feedback!
Known issues for this release:
Since no mechanism exists to ensure mutt has already opened the temporary message file, it cannot be safely unlinked. This is a limitation of AppleScript's interface to the shell via Terminal.app. However, since the file is created in the user's temporary files directory, the files should be cleaned periodically.