Symptom: All your contacts have disappeared from your iPhone and perhaps from your address book, and MobileMe contacts as well.

Cause: Updating the MS Office software on your Mac. For some reason certain patches of MS Office will cause an issue with your sync services.

The Fix: This is specific to Macs running MS Office:
1) Launch Outlook on Mac and under Preferences, de-select Sync Services.
2) Close Outlook and then re-open.
3) Re-enable Sync Services.
4) Wait.

Your contacts should start appearing in your address book, iPhone contacts and MobileMe contacts. This can take anywhere from 30 seconds to 5 minutes depending on your connection speed.