Can anyone think of a synonym for "shit eating grin" that would be usable in polite company?
This definition from the OED captures the meaning I'm looking for perfectly:
A broad grin expressing uncontrollable delight or (self-)satisfaction, esp. characterized by awareness of having got away with something normally considered outrageous or improper.
As pointed out in this Metafiler comment, an important nuance is that this sort of grin is uncontrollable.