The one problem I’ve found when upgrading to Lion is that suddenly iCal couldn’t sync to my Google Apps Calendar account – instead it repeatedly asked for the password. I’m still not really sure what caused this, but my solution was to simply delete both ~/Library/Preferences/*iCal* and ~/Library/Calendars. You really only want to do that if you exclusively use Google Calendar. If you have local calendars deleting ~/Library/Calendars will delete them.
Once I’d done that, I re-enabled the Calendars option for the account in System Preferences, “Mail, Contacts and Calendars”, entered my password one last time and it worked perfectly.