On a GPS forum which I visit people having been saying Garmin is behind on the OSX port. However, if they continue to go in the same direction as their newer products then it may be a moot point. As I understand, the Nuvi series can be mounted on your computer via USB like a flash drive. All the data is stored in GPX files which can be exchanged with other applications or modified with an XML editor. Maps and software updates are just files which can be copied to and from the GPS. This open source program runs under OSX to exchange POI's with the Nuvi:
http://www.hearsay.demon.co.uk/mac/MakeGPI/index.html. I haven't seen a Nuvi yet, but it sounds like Garmin is finally bringing the GPS out of the old 9600 baud serial port days
Of course none of this helps people like you and me with older Garmin GPS'es, so hopefully they will keep their promise an deliver an OSX version of MapSource this year. I have the eTrex Legend C, and yep the USB interface is a big improvement over the slow serial ports. I think mine is the same as the Vista C but without the compass and altimeter (or something like that?). But I also believe the newer "x" versions have removable SD memory cards. I'm not sure what can be stored on the cards... is it just maps? Or can you have a different set of waypoints on each card? Or, for that matter, does the size of the card have something to do with how many waypoints you can store?