I use BootCamp on my MacBook Pro to interface with my Garmin GPS. And yeah, it would be great if Garmin ported Mapsource to OSX soon. They were supposed to roll out OSX support in Q4, 2006.
I haven't used this software, but you could try checking out
MacGPSPro . It's supposed to work with many Garmin GPSr's and OSX.