This isn't exactly specific to Pentax, but it is marketed on their website to Pentax SLR and DSLR owners, so I thought it might be worth keeping tabs on. For $29.95, the circus sounding "Professor Kobré's LightScoop™" is a satisfying-looking little doohicky that you can use to bounce your pop-up flash (ie. get more diffused shadows, control light direction, etc.).
I've played around with ideas like this in the past. At one point I made a pop-up bounce out of a toilet paper roll tube, and later on tried using a white microphone foam wind cover pulled over the popped-up flash on my *ist DS. Surprisingly, both worked at diffusing alright, but I think the LightScoop looks a little more versatile if you're looking for something that won't make your subjects laugh at you and your camera. But unfortunately it doesn't dispense any Pez. (via Digital Raingefinder)
If you're looking just to diffuse light from your pop-up, a little less expensive option is to try using a transparent film jar, coffee lid, construction paper, vacuum bags, or a sliced ping pong ball. Or for a few bucks you could get the LumiQuest Soft Screen Flash Diffuser for Pop-Up Flashes. It looks like a sail on your DSLR, but the 4"x4" screen is supposedly very good at eliminating hotspots from the flash on close-range subjects. It comes with velcro attaching strips and retails for about $8 to $10 U.S.
Links: LightScoop, LumiQuest Soft Screen Flash Diffuser for Pop-Up Flashes, Do-It Yourself Pop-Up Flash Diffuser Link A, Link B