Not that I am aware of. You could, of course, run Windows on it and load Garmin Mobile PC and use the Mac as the nav unit.
I have a garmin nuvi. Is it possible to plug it into the Mac and send it destinations, add way points, ect all without having to do anything on the device itself. We are going on a roadtrip soon and we are bringing our Mac for my girlfriend to use to navigate and such. It would be great if we could keep the garmin plugged into the Mac so she could send over destinations on the fly. If it's not possible on the garmin is there possibly another way of going about this?