I, too, have been annoyed by the lack of GPS software for the Mac,
So I'm writing one.
CacheKeeper will run only on OS X initially, and if I get enough support, I'll release Windows and Linux versions as well.
CacheKeeper will be very Geocache-friendly. The initial list of features is: ability to save waypoint lists, logging of waypoints you've nabbed, drag-and-drop of .loc files into the app for immediate upload to your Garmin, and maybe the ability to auto-create routing for a bunch of waypoints. The goal is to make getting geocache waypoints into and out of the gps without relying on VPC.
I'm early in development now, but hope to have at least a beta out sometime in October. So far I'm talking happily to my GPS III Pilot and GPS 12 over a Keyspan serial adapter. Thanks to Garmin's developer-friendly attitude, I plan to support all Garmin GPS units available at the time the app is released (however I'll really depend a lot on beta testers to provide feedback for units I don't have access to).
So that means I could use some beta testers.
Let me know if you're interested. When I can find the time I'll throw together a registration page and progress page. For now, the basics can be found at geocache.aviate.org. Links will appear sooner or later on that page for more detailed info.
-PilotMike-
Programmer/Pilot
Apple Computer