I have a MacBook Pro running Snow Leopard and a MacBook Air running Lion. Once I migrated my MobileMe account to iCloud, my iCal data would no longer stay in sync. I thought I was out of luck until I found a fix from Damien Barrett. In fact, it’s simple to get iCloud calendars working with iCal on Snow Leopard.
As Damien explains, you’ll have to first delete the old account, and then create a CalDAV account that points to p06-caldav.icloud.com as the server. Once you’re done give it a few moments to sync and you’re back in business. Read Damien’s post for all the details. I’ve done it and it’s working perfectly.