A search for "frog" on Snopes leads one to comments of Dr Victor Hutchison, a zoologist at the University of Oklahoma, who assures one that the "boiled frog" claims are just an urban legend and contrary to fact.  A Yahoo search on "victor hutchison" frog finds many more references to his work.  But one would still like to see an actual published writeup of an actual experiment.  In about fifteen minutes of poking here and there I found no reference to such an experiment.