I use iCal on the Macs as an organiser and have 4 calendars running plus a subscription to one on my wifes computer. These are all synced via Apples MobileMe service to the Calendar app on my iPhone.
I recently noticed that any events created on the iPhone were not being synced to MobileMe or into iCal on the Mac. Events created in iCal were being synced to the online calendars on the MobileMe web interface, but not to the iPhone.
Plus I now had 3 calendar instances on the iPhone, All Calendars, Calendars on the iPhone and Calendars at my MobileMe account. And many events were duplicated.
I reset the sync history on the Mac, and re booted the iPhone to no avail.
A quick search of the forums on the Apple website found the answer.
Turn off calendar sync on the iPhone, wait a few seconds and turn it back on.
Voila, all back as it should be with one set of calendars reflecting those on the Mac, and events sync almost immediately in both directions again, as they should.