I've been waiting for the Mac OS X compatible AvantGo software forever now. You are not alone!
They seem to have cut the R&D cost for comsumer services. The one for OS X is not out yet. No third-party apps are available. I wonder if they'll ever release one. Since the release of the OS X a couple of years ago, they keep saying just install the Palm Desktop V3 and use it from Classic. Unacceptable. I'm almost giving it up now...
And the beta for Palm OS 5 is currently only available for Windows.
So if you are on Mac X and Palm 5 you are really screwed... like myself
I have a CLIE NX70V.
I use my Vaio just to sync my AvantGo these days. Like pimentoLoaf has suggested, using Virtual PC is another way to go.