If a Mac wont run Lion, then Apple wants you to buy new hardware.
Like it or hate it, but that decision has been made at Apple obviously, because no update or backport that would enable older OS to participate in iCloud is available.
iCloud is baked deeply into iOS5 and Lion, both are shared by the same codebase which is Darwin version 11 , more or less Lion for Desktops and Lion for iphones if you want so. Either there is a technical thing that demands this specific OS version or it is a marketing decision. At least Apple markets iCloud as a feature of the new iPhones/iPods, so they see it tied to minimum iOS5 which itself is like I said the "slimmed down" mobile Lion OS.
Concerning SnowLeopard or even Leopard Apple is absolutely silent when it comes to iCloud.