Here's an important warning about Address Book and iSync: Make sure there's no pairing between Adress Book and your phone when you perform synchronization using iSync thru Bluetooth for Symbian OS mobile phones.
I use my phone to send SMS with my PowerBook paired via Bluetooth with my mobile (turn on both Mac and mobile Bluetooth, pair them, launch Address Book, click the Bluetooth icon button on the Address Book; then you can click on any mobile phone number at Address Book to send SMS and dial from Mac).
But one day, I forgot to disconnect from Address Book and then performed a data sync using iSync. After it was complete, all my records were gone! Luckily, I did backup regulary.
[robg adds: A previous hint covered Address Book data loss when syncing and modifying contacts. As always, these incidents point to the importance of good, recent backups. Even if you don't back up all your data files, at least use Address Book's own File: Back up Address Book menu item to copy your contacts to a safe place every so often.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2005081815175480