If you have a .Mac account using Panther and think you can get by without upgrading to Leopard (like you did with not upgrading to Tiger the last time), this time you’re SOL. You’re going to have to bite the bullet and bump up to OS 10.5, or at least the latest Tiger update, if you hope to continue using the US$99 service.

Starting last Friday, Apple began the process of abandoning full .Mac syncing support for users of OS 10.3. To have “uninterrupted service”, Panther users will have to pony up for at least 10.4.10 or later.

A technical note from Apple states that they are doing this to support .Mac enhancements that will emerge with the release of Leopard today, and will leave users of older versions out in the cold, especially with Calendar syncing.

Apple says

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.”

Thus pushes the inexorable march of progress.