Well, you can do a nasty double-read-only sync: sync GPE to Google Calendar, publish iCal to iCalX, import iCalX into Google Calendar. Then you have essentially two calendars - a "created on n800" calendar and a "created on mac" calendar. (Found this on google when I wanted to know the same thing... not wanting to take credit for the idea) I have no idea about contacts... OpenSync might have something to say though...

(to answer your question - KDE uses OpenSync in kitchensync, and bluetooth is just another way to establish a connection and doesn't inherently gain us anything. I wish this was easier, but at this point I think contributing to OpenSync is the best course of action. Apparently the current development version has real promise, and even the "old not as good" version works for my daily use, at least when I have more than my MacBook...)