The feature (or platform, or...?) was discovered by French blog Consomac, which had stumbled across the reference to a "Castle" while digging for clues about iCloud and a Find My Mac feature in the next big OS update from Apple.
Instead, they found the Castle. At first glance, it appears to be related to MobileMe. As you can see in the screengrab, the system asks the user to "complete you Castle upgrade" and "Click OK to open System Preferences and complete your upgrade from MobileMe to Castle."
The name Castle is likely (definitely) a placeholder code name for whatever cloud-based platform Apple is preparing to launch (or preview?) at WWDC. It implies safety and security though, no? Or war and a prolonged siege as hackers and marketers try and obtain your valuable purchasing information. One or the other. Or both! [Consomac via TUAW]