I think I do know. Calendars, contacts, mail and notes will work on older devices, and even on non-Apple devices, using IMAP, SMTP, CalDAV and CardDAV (or that other one, but I think it was CardDAV). Here's a pic (I've shown it in
another thread as well):
I currently have these iCloud services working on my iOS 4 iPod touch. Photo Stream, on the other hand, doesn't use functionality already present in iOS 4, and thus is only going to work with iOS 5 devices.
Also, the above mentioned features should work on older versions of Mac OS X as well, and possibly iOS 3 or even iOS 2. Theoretically, all devices that support MobileMe should be able to be configured for iCloud.