Garmin has released a Mac beta version of their WebUpdater for handheld GPS units. All it will do is update your firmware to the most recent release, but that's better than nothing. If you want to transfer waypoints or routes, it still looks like you'll have to use GPSy®.
Unfortunately, it truly is a beta, the WebUpdater crashed when I tried to update my Nüvi 660.
Updated 2007.04.17: Allory Deiss let me know that v18.104.22.168 beta was released last night. I tried it out with my Nüvi and it worked!
Updated 2007.04.18: Actually, I spoke too soon. The Garmin webupdater worked -- but trashed the map files on my Nüvi. I had to send it back to Garmin to get it fixed. So don't use the alpha version of the Mac webupdater with your Nüvi units!