Harkening back to a , allow me to reiterate that Apple's syncing strategy needs a stern overhaul. When you throw MobileMe, Google, and iTunes syncing into the mix and aren't aware of how they interact it's easy to find yourself in this kind of situation. Let's see if we can bring some clarity.
First, you're in danger of getting duplicate items if you've switched on calendar syncing with both MobileMe and iTunes. To check, attach your iPhone to your Mac, select it in iTunes' Source list, choose the Info tab, and peer at the Sync iCal Calendars entry. Look for these words:
This is a very strong hint that you're doing The Bad Thing and that you need to make a choice between syncing with your Mac or with MobileMe. You can disable calendar syncing within iTunes by unchecking the Sync iCal Calendars option. When you do, iTunes will offer to remove its calendar entries from your iPhone. Or, turn to your iPhone, choose Mail, Contacts, Calendars; select your MobileMe account; and flick the On/Off toggle switch next to the Calendars entry to Off. You'll be offered the option to delete these calendar events from your iPhone.