Just fixed mine, with similar problems. Wouldn't sync with iphone and OF. This made me suspect a iCal problem. I reset iSync, then followed the directions for reconstituting the Calendar folder in ~library/calendars (they say drag it to the desktop). Everything magically started working again. Don't forget to back up!
http://docs.info.apple.com/article.html?artnum=306537
BTW, to the person who wonders why to use the talkbar of iCal: that's for setting the GTD-discouraged alarms that I still need. Alarms can be set from tasks, or from events., and probably some way to even do it from mail.
BTW, all this in Leopard.
Good luck. When OF, Mail, iCal and the iPhone all work together, it's heaven!!!