iCloud: Features that work with one account at a time on iOS 5 and OS X Lion or later

You can use iCloud Mail, Notes, Contacts, Calendars, and Reminders with multiple iCloud accounts on iOS 5 or later and OS X Lion or later.   iCloud Bookmarks, Photo Stream, Documents in the Cloud, Backup (iOS), Back to My Mac (OS X), and Find My iPhone, iPad, iPod touch, or Mac can only be used with one iCloud account at a time (the "primary" account).