Ok, this totally resolves my iPhone sync issue with iCal and I can certify that it is the solution for Plaxo Preview users:
Reset your truth database by launching iSync (yes, iSync) and
opening the Preferences… dialog. Press the Reset Sync History… button
and synchronize again.
This will leave your clients registered and your data intact, but
force a first-time slow synchronization of all of your sync clients,
and should result in the transfer of your iCal data to the iPhone. You
should back up your iCal data using the built-in option to do so
labeled Back up Database… under the File menu of iCal before performing
There's no way I would have come up with this myself, so all of the credit goes to Geeks R Us -- you can find a link to their solution page below.