Yes, it supports Panther. They were very fast to react to the release of Tiger and supported it from the release. It seems Tiger broke a few applications -- Popcorn is one that comes to mind. Left me waiting for the patch!
Mark/Space has been a long-standing supporter of the Mac platform and a good developer. The integration between the Treo and the Mac is flawless with Missing Sync. If you purchase an SD card (which is recommended), you can and should store your music on the SD card. This is made so much easier with Missing Sync because your Treo will appear in iTunes. Then it's as simple as drag and drop. Here's a screenshot:
I think I might go ahead and set up the Mac & Treo forum at your recommendation.
-Chris