Well, you can't see the P900 on your desktop as far as I know. In theory, as P900 supports ftp over BT this might be possible in the future. There is a 'Browse Device' option in BT under Panther which successfully establishes a connection to my P900, but never actually shows any files.
If you have a memory stick/media reader reader (PC card, or USB), you can certainly mount the memory stick. I do it on my PowerBook all the time.
Microsoft Virtual PC 6 basically creates a PC inside your Mac and then fools all software and USB devices into thinking it is a real PC. Excellent piece of software and allows Mac users to fully communicate with the P900. The only bad thing is the £200 price tag.
However purchased this last week and have no regrets at all!