You can 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
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.)