Multiple .Mac users report receiving emails from Apple regarding a .Mac sync update scheduled for today.
Dear .Mac member:
Due to improvements being made to .Mac Sync on October 19, 2007, please note the following changes.
Mac OS X version 10.4 Tiger users:
To ensure uninterrupted service, you must be using Mac OS X version 10.4.10. To check which version you're running, select About This Mac in the Apple menu. If you are not running version 10.4.10, click the Software Update button and install the update to 10.4.10.
Mac OS X version 10.3 Panther users:
To accommodate new enhancements, .Mac Sync will no longer support syncing of calendars between a Panther-based Mac and a Mac running Tiger or Leopard. Syncing calendars between Mac computers running Panther is unaffected.
These changes will help enable .Mac Sync to do more than ever. Thank you for your patience, and thank you for being a .Mac member.
The .Mac Team
The update is further explained on this Apple Support page. The changes would seem to be in preparation for syncing improvements introduced in Mac OS 10.5 "Leopard".