To ensure uninterrupted service, use Mac OS X 10.4.10 or later. To check which Mac OS X version you’re running, select About This Mac under the Apple menu. If you’re using Mac OS X 10.4 through 10.4.9, update to Mac OS X 10.4.10 or later.
If you use Mac OS X 10.3.9 or earlier, be aware that in order to accommodate new enhancements, .Mac Sync will no longer support syncing calendars between a Mac OS X 10.3.x-based Mac and a Mac OS X 10.4.x-based Mac. Syncing calendars between two Mac OS X 10.3.x-based Macs is unaffected.
These changes will help enable .Mac Sync to do more than ever before.
More info here.
MacDailyNews Note: Mac OS X 10.5 Leopard will feature the ability to sync Dashboard widget preferences, Dock items, application and system preferences, and Mail notes across all your Leopard-based Mac computers.