You can migrate your MobileMe account to iCloud at http://me.com/move even with Snow Leopard - when asked to confirm that you have Lion on your Mac, just lie and say 'yes'.
If you have MobileMe mail set up to collect mail from external POP accounts you should cancel this before migrating, or you may find it still working in iCloud (where it isn't supposed to) and with no way of stopping it. You may find the MobileMe email settings will continue to work for a time after migrating but it will stop eventually so you should not rely on this. You should delete your MobileMe email account from Mail.
You can then set up Mail manually in Snow Leopard to access iCloud email; it's a slight fudge because the 'wizard' will attempt to connect you to MobileMe as soon as you enter an @me.com address. The process, which includes a workaround for this, is described here:
http://www.wilmut.webspace.virginmedia.com/notes/icloudmail.html
You may be able to sync your calendars to iCloud by means of an unsupported hack, and there is a convoluted hack which has been reported as working for Address Book (and as not working by others - I've not tried it), but iCloud's other facilities will not work. This page outlines the situation when you migrate to iCloud, including a link to the calendar hack:
http://www.wilmut.webspace.virginmedia.com/notes/icloudSL.html
You can set up Mail on Leopard using the method linked to above but you will not be able to sync calendars or contacts with iCloud.
There are a couple of third-party solutions which may be of interest.
- BusyCal is an iCal-like calendar application with extra facilities: it can sync with the iCloud Calendar while running on Leopard, Snow Leopard or Lion. A single user licence (two machines allowed) is $49.99.
- Soho Organizer can sync Calendars and Contacts with iCloud on Leopard, Snow Leopard and Lion. A single user licence (multiple machines allowed) is $99.99.
(I have no connection with either firm.)
I would have expected that you would be able to sync older devices using iTunes, but note that if you do this, all devices have to be synced using iTunes as the calendars and contacts have to reside under 'On My Mac' - you cannot sync calendars you are syncing to iCloud using iTunes, so it's one or the other.
There is no Keychain syncing.
As you will be aware, the iDisk will be removed and its contents deleted. This page examines some third-party alternatives:
http://rfwilmut.net/migrate3
The Backup application will sync to local disks or DVD but AFAIK will not sync to any online service other than MobileMe.