Mac OS X 10.6.9 will support iCloud, evidence spotted by a developer would suggest.
MacRumors reports that a developer upgrading his MobileMe account to iCloud on a Mac running Mac OS X 10.4 Tiger saw a warning message telling him that he'd need to be running Mac OS X 10.6.9 or later.
The messages read: "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".
The message seems quite surprising as Apple has emphasised that many of iCloud's features would require Mac OS X 10.7 Lion, though as MacRumors notes it has been coy about an absolute set of minimum specifications for iCloud.
So it looks as if we can expect a new update for Snow Leopard in the near future, though whether it will come before or after the public launch of iCloud is unclear.