I can only remember encountering this problem when some other application was syncing with iCal, or when OmniFocus was being synced with iCal on multiple Macs. (vs. syncing OmniFocus with iCal on one Mac, then letting MobileMe sync carry those calendar events to the other Macs.)
That said, if you email our
support ninjas, we'll be happy to investigate what's going on here with you. Sorry for the trouble!