According to a image sent in to MacRumors, iCloud support may be coming to Snow Leopard users. As you can see in the screenshot above, the .Mac preference pane in a previous OS X's System Preferences reads: "You will no longer be able to sync with this machine because you've upgraded to iCloud. iCloud requires a computer running Mac OS X Snow Leopard v10.6.9 or later for Contacts, Calendars, and Bookmarks."
This message appeared in OS X 10.4 Tiger after a developer updated his MobileMe account to an iCloud account. The notice appears to be the first indication that OS X 10.6.9 is in the works. Presumably it will only include minor bug fixes and provide limited iCloud support for Snow Leopard. Currently Apple states that iCloud will require OS X 10.7, but that is presumably for the more advanced Documents, iTunes Match, Photo Stream, and Backups features. It's reasonable to assume Apple isn't going to lock non-Lion users out of their @me.com email addresses.