It took me about ten minuites to set up iCloud sync on my iMac, Macbook Air (both using 10.6.8) and new iPhone 4S. The details are onr the web and the key points seem to be that you first activate iCloud through your iPhone and after that you can set up a caldav server in iCal using your apple id as user name and password and the server address as: 'p06-caldav-icloud.com'. I had to do this twice on the Macbook Air but it took only one try on the iMac. You know you've succeeded when the new iClound calendars Home, Reminders and Work appear in your iCal. The Reminders calendar syncs with the Reminders app in the iPhone 4S
I think Apple has made it possible to use iCloud in 10.6.8 but just hasn't told us. One caveat I would add is that I've only used it for iCal. Not for mail, documents, photos and suchlike. Maybe there is just limited functionality. Whether there will actually be a 10.6.9 update is an interesting question. My money is on 'no'