I just wrote an email to the trolltech guys who work on the QtopiaDesktop software to inquire about any possibility to have Qtopia syncing with OS X PIM applications (Addressbook, iCal) in the next release and here is his answer:

"(me) Therefore I would like to see in the next version
implemented a feature which would allow QtopiaDesktop to synchronize
with the default PIM applications under OS X (Addressbook, iCal). I
think it could be a plugin to do the job and the synchronization
procedure could be: sync zaurus & qtopia, then sync qtopia & mac and
vice versa.

(QT) This is on my list of things to do but until a customer requests this
functionality it will remain a somewhat low priority. Qtopia Desktop
has the ability to support third party plugins although their creation
and use are not supported by Trolltech. Again, I hope to change this in
the future but it is not a high priority task. "

So, if someone would like to have this write a support mail to info@trolltech.com.

P.S. QtopiaDesktop works only with .xml base PIM on Zaurus (initial ROM in 5xxx series and A300).