I experienced this fault some time ago and just changed the sync settings in itunes to say I didn't want to sync contacts notes or calendar with the phone, then synced the phone (effectively removing anything that had been previously synced).
I then disconnected the phone, reconnected it again, went back to itunes and ticked the sync options I was looking for and synced again. This sorted it out.
Ultimately, however I wanted two-way syncing with outlook over the air (i.e. without relying on itunes) so updates to my calendar made while I was out and about were in my outlook calendar (almost) immediately. I solved this by installing Google Calendar sync on my computer and pointing it at a google account, then setting up the iphone to sync with this same google calendar. Works a treat and is completely free