Give this a try. From the .Mac discussion boards:
In certain circumstances, no amount of troubleshooting seems to correct an iSync 1.x-related issue. If you have found this to be the case, the following information should cover everything that you can do to reset iSync, completely removing all traces of it from your hard drive prior to reinstalling it:
After launching iSync, choose Reset All Devices from the Devices menu, and be sure to select "This Computer" from the pop-up menu. Then, quit iSync.
If you had ever installed iSync Beta, download/run
iSync Uninstall 1.0 to remove any traces of iSync Beta from your hard drive; otherwise, skip to step 3 (note that using this iSync Beta uninstaller may remove certain components described below)
Delete iSync.pkg from /Library/Receipts/
Delete the iSync application from /Applications/ (if present)
Delete com.apple.isync.plist and com.apple.isync.registry.plist from /Users/username/Library/Preferences/
Delete /Library/Application Support/SyncService/
Delete /Users/username/Library/Application Support/SyncService/
If you have used a Palm OS device with iSync and/or have installed the iSync Palm package, delete the following files:
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/Library/Receipts/iSync_Palm.pkg
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/Library/Application Support/Palm HotSync/Conduits/Apple
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/Users/USERNAME1/Documents/Palm/Users/USERNAME2/Conduit Settings/iSync Conduit Settings
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where "USERNAME1" is your Mac OS X username on your Macintosh, and USERNAME2 is your Palm OS device's username
If you have used the Address Book Sync functionality of the .Mac Website, login to the .Mac Website, click on Address Book, then click Preferences, then uncheck "Turn on .Mac Address Book Synchronization"
Download/install iSync 1.1 from
http://www.apple.com/isync/
Perform a sync between your devices, but do not register your computer with .Mac yet (if your computer already shows as registered, unregister it before you perform your first sync)
Register your computer with .Mac, preferably under a different computer name than was used previously
Perform a sync between your devices and .Mac