Support OS X Gatekeeper
Tor for Mac is currently not signed using a Gatekeeper certificate.
This means users can only run the app on Mountain Lion by right-clicking. I also suspect that by the time the next version of OS X comes around,almost anything is signed and the dialog shown for manually running unsigned apps will look rather scary.
These certificates are available for free from Apple.
Besides being good for Tor users, this is also an experiment to see if Apple dares to abuse their power and revoke the certificate at any time. This has been suggested by various people. I'm confident that they won't, especially since there are legitimate reasons to use Tor just as with Transmission.app (a BitTorrent client) which has already been signed.