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, but you cannot sync Contacts or Bookmarks and 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