The iPhone resets contacts, i.e.; the contact list goes blank (displays zero contacts), your favorite list reverts from names to phone numbers, and your ringtone assignments all reset to the default iPhone ringtone.
-The phone contacts will re-populate after a minute, but your ringtones assignments will be reset. (The ringtones are still present, just not assigned individually anymore)
also, some duplicate or deleted contacts may return.
MobileMe. A glitch in syncing causes the iPhones address book to reset to Apple's server listing, which doesn't save ringtones, and results in all contacts reverting to the default tone.
Until the issue is corrected, turn off syncing contacts through MobileMe. (Use iTunes for syncing)
1) Hook-up the iPhone, and backup with iTunes. (Make sure iTunes actually backs-up your phone. -Quit iTunes and re-run if it doesn't backup automatically.)
2) On the iPhone, click 'Settings'.
3) Click 'Mail, Contacts, Calendars'.
4) Click the MobileMe account to display the configuring options.
5) Click the 'Contact' slider to OFF, and confirm when prompted.
6) Rerun iTunes, and sync your iPhone to restore your contacts.
7) Assign those ringtones ! )