I upgraded to iCloud, as you would and discovered that one of my machines running 10.6.8 would no longer sync with iCal. There is a solution: add iCloud as a CalDAV account. In iCal, go to Preferences>Accounts>+
Use the server p01-caldav.icloud.com and add your iCloud credentials as User name and Password. This is working for me.
Turned out to be a pain to stick with this especially since not everything got synced as expected. I ended upgrading to 10.7