iCloud Beta is now open to developers using iOS 6 beta. The site is located at beta.icloud.com.
According to what we’ve read, the calendar functionality is updated, and the Find My iPhone App has been given a redesign, and has a cool battery status indicator for when your device is lost. There are also some other updates like new Notes and Reminders apps that work just like the iOS versions.
We can expect to see it this fall with iOS 6.