There's a forum for iCal syncing to OmniFocus here:
http://forums.omnigroup.com/showthread.php?t=6648
It worked one time for me. I tried it out the first time but I disconnected it about a year because I wanted to keep my tasks and calendar separate.
Earlier this month, I tried it again and it just wouldn't seem to sync....
From what I've heard, iCal syncing seems to be a moving target. it works for some but not for others.
The methods suggested in the forums just didn't seem to work this time.... *sigh*.....