I've never refrigerated ketchup, but went looking for some expert opinions. The third post here is interesting:
http://msgboard.snopes.com/cgi-bin/u...8;t=000396;p=1
To summarise, Heinz say you don't need to but refrigeration keeps the flavour better, while some Food Safety Specialist bint reckons that everyone's wrong and it can spoil if you don't. I reckon her kids play in a sterile bubble with glass-coated toys.