With Mac OS X 10.4.1 or later, the first time you add a mobile phone to iSync version 2.0, the default first sync option is "Merge data on computer and device," which doesn't erase anything from your phone. You can download and install Mac OS X 10.4.1 Update or later from Apple Downloads.
With Mac OS X 10.4, the first time you add a mobile phone to iSync version 2.0, the default option for the first sync is "Erase data on device then sync." This option does just that—it erases syncable data from the phone, no matter what's on it. Change it to "Merge data on computer and device" if you wish.
If you don't want to lose the data on your phone, use "Merge data on computer and device." This setting won't erase anything from your phone.
Important Bluetooth note: When you add a Bluetooth phone using the Bluetooth Setup Assistant, iSync opens but will not show you the default first sync setting ("Erase data on device then sync") unless you click the phone's icon in the iSync window. Be sure to double-check your iSync settings before pressing the Sync Device button to avoid data loss.