When I first opened OmniFocus and tabbed to the Sync pref pane I saw a slew of old, previously deleted calendars. After a little digging I found this article outlining how to reset the SyncServices folder
and that cleared up the old calendars problem. For others having this problem all you do is open terminal and paste in this command.
Unfortunately after clearing out all the kruft, OmniFocus won't sync. Nothing shows up in iCal. I have no idea if this issue existed before I reset syncservices since I didn't try beforehand, but I thought I'd include it just in case.
I'm also running Spanning Sync (which syncs iCal calendars to Google Calendar) and it's had its share of issues in 10.5 as well, but seems to be functioning fine right now.
EDIT: I uninstalled Spanning Sync. No change. Then I ran across this forum post
which recommended assigning all contexts to a calendar. Still no change. iCal refuses to sync and I get no error message at all. OmniFocus seems to think everything is fine.