iCal sync is a different code path than the OmniFocus sync, so your settings in the Sync preferences pane aren't likely to be relevant. We're syncing through a piece of OS X called Sync Services - for some reason, it looks like it can't or won't send us a list of your calendars.
Best thing to do now is probably contact our
support ninjas via email so we can investigate this with you. Sorry for the trouble!