Garmins do have USB ports. My 60csx is at present connected to my Mac via USB. I can download geocaching waypoints directly from the Internet with Geocaching Menu. I can transfer tracks or routes to and from the 60csx with LoadMyTracks. I'm trying out RouteBuddy which is a promising new mapping program for Mac. And if I turn on USB mass storage mode on the Garmin, it appears on the Mac desktop like any removable drive.
All this works straight away: no drivers to install, no hassle. For firmware updates and transferring maps via Mapsoure I still need VirtualPC and Windows, but Garmin have announced that they will be bringing out Mac versions of their software towards the end of the year: let's hope we don't have to wait too long.