i for one would also like to know which diffuser would be best for the built-in popup flash. while i do know how much better an external flash is (coming from a full nikon system), i would like to go around with the K20D + pancakes only most of the time and just use the popup flash for those moments i need a bit more light.
there's the lumiquest, and there are some DIY ones like the film canister:
Matt Greer Photography: DIY Lighting Equipment
i hope someone can comment on their experience with these thingies for the built-in flash