A developer informed the site that after upgrading to iCloud from MobileMe, the following message appeared in his .Mac preference pane in Tiger.
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.
Currently, Apple is seeding developers with Mac OS X Lion 10.7.2 which is required for iCloud. However, it seems that at least some iCloud support will be coming to Snow Leopard.