OS X 10.5 (Leopard) now allows for the "message:" url type. A number
of mac-only productivity apps have been using this to link mail
messages to "action items" (i.e. omni focus).
It turns out this isn't so hard in emacs either. You need a little
lisp and a little Applescript, and then you are done - I can now be
reading mail in Mail.app, press "F5", and have a link to the mail
become an interactively prompted task. Similarly with Safari, Finder,
or probably anything else that is Apple-scriptable.
I won't garauntee these won't cause you problems, but thought I'd
share to see if anyone had any improvements.