If you move your exisiting MobileMe to become icloud , all machines that can't run Lion will be out of the syncing.
And no, Snow Leopard does not support icloud. Rumors are rumors. But if something like a 10.6.9 was in the works, we would have heared or seen more then a photoshopped screenshot .. So : No. Lion only. 10.7.2 to be exact.