From The Guardian, via my man Larry K.:

The description of the French as "cheese-eating
surrender monkeys" was not coined by Bart Simpson, as
we suggested in 'Wimps, weasels and monkeys - the US
media view of "perfidious France"', page 3, February
11. It comes from the Simpsons character Groundskeeper
Willie, the Scottish immigrant who takes care of
custodial matters at the elementary school.