%@ is the new name of Apple’s online service (was .Mac)
This string, found deep within the code for iCal, makes it pretty clear that Apple plans to make some changes to .Mac, its online service, um, service. The $99/year service gives people e-mail, an easy way to share content, sync information between computers, etc. It’s sorta outdated, especially in today’s environment where Google Apps can some of the same stuff for free.
Some ideas that have been thrown around: over-the-air calendar, email and contact syncing; an all-around less expensive .Mac service, possibly with the purchase of an iPhone.
Personally, I never saw much use for .Mac, but maybe this new version will change my mind. We’ll find out for sure by WWDC.