Hey thanks! I have to say, the Shard of the Cudgel was one of my favorites!
But I too really liked the suprise factor. And I think it is a theme that could be used in GH a great deal. The world is fairly segregated, with only a few places, Greyhawk City and Highfolk for example, being integrated. But imagine the possiblities.
A few years ago Dragon issued some articles on two low CR creatures who, when paired, became VERY difficult. That is the basic concept behind a D&D party, and I think it should be the premise behind more monsters.