I just talked with an Apple Care rep and I was told that I can update my MM account to iCloud and my calendars and email (@mac and @me) will still sync with my Mac running Snow Leopard. I only asked about those 2 specific things, because that is all I care about. You will have to do the migration to iCloud through your browser
Just to be clear. The current answer is NO, iCloud will not work with Snow Leopard.
I am not happy with this answer, but that is what it is. I think I am going back to iOS4 if I can.