.Mac's calendar feature has a limitation that does not permit you to access your iCal calendar, add and make changes, and finally synchronise/insert them into your iCal desktop calendar you keep back home. There is a way around this -- it's not perfect, but it works.
Publish your iCal calendar to a server (see this article on Apple Support). By using Firefox's calendar feature, which can read iCal .ics files, you can access your calendar from any computer. If you delete the published calendar in iCal (after you backed it up and chose not to delete the remote version) and subscribe to the calendar you have just published, you can work with the same version of your calendar both on the web and your desktop.
It is not a proper synchronisation, but if you remember which one you have worked with last, you can make it work if you stick to manual update in both the web version and your iCal version.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060315123230728