I just noticed this myself and for grins I opened the Date & Time preferences, made a nominal change (set my nearest city from Austin to Chicago), unchecked and rechecked the date & time sync box and my iCal icon went from Nov 19 to Nov 20. I have no idea if it was just opening the preferences window or making the change but it looks to be fixed.
My icon was one day behind today (showing Dec 14 in the dock, 15 while running, and then 14 again after quitting iCal.app). Changing the date-time preferences like someone suggested did not work for me, but a "killall Dock" did solve the issue.