If you are a MobileMe customer and want to sync your contacts and calendars Over-The-Air (OTA) to your iPhone there's an easy and a hard way to do it -- and the easy way isn't exactly intuitive or documented very well. This post will explain how to setup up MobileMe the easy way so that you don't have multiple versions of your contacts and calendars on your iPhone.
If you've already synced your iPhone to iTunes and checked the boxes for syncing your contacts and calendars, I recommend starting over -- which means restoring your iPhone as new. First make a solid backup of your iPhone and be sure to sync any photos you've taken to either iPhoto or Aperture as they get wiped on restore.
Also, make sure that any notes you've taken using the iPhone's Notes app are synced to Mail, which requires 10.5.7. If you've got data or levels stored in your apps, follow the developer's instructions for backing up your data as this too gets lost after a restore. Once you're confident that your data and settings are backed up, connect your iPhone via USB to iTunes and click iPhone > Summary > Restore. Once your iPhone is restored resist the urge to restore from a backup and instead set up the iPhone as new.
Make sure that your account information is correct in System Preferences > MobileMe, check the boxes for whatever you want to sync under the Sync tab (I sync bookmarks, calendars and contacts) and check Synchronize with MobileMe Automatically. Click the Sync Now button and let it do its thing. Once syncing has completed, log into Me.com from a Web browser and double check that the data on the clould is accurate.
When you're confident that the data on MobileMe is accurate and tight simply add your MobileMe account information on the iPhone itself in Settings > Mail, Contacts, Calendars > Add Account... When completed I also recommend throwing the switch for "Find My iPhone" as you never know when this is going to come in handy.
Let your iPhone sync with the could and all your contacts and calendars will automagically appear on your iPhone. If you subscribe to family or third-party calendars (like holidays, etc.) you can add these directly on iPhone too.
The trick here is not to sync your contacts and calendars via iTunes using the dock cable -- which is counter-intuitive. If you do, it can be difficult and confusing to remove them from your iPhone and adding a MobileMe account post-sync can be a nightmare. The best method is to start with a new iPhone and add your MobileMe settings directly onto the iPhone directly.