ecce38,
Don't panic. As long as you change lenses, your going to occasionally get dust on the sensor. There are some things that help before running to the local camera tech for 40-60 buck cleaning.
1) Do a mirror up sensor cleaning command. Take the lense off with the camera body opening pointed down. Hold the body in this position close to (but not directly over) a light source so that you can see the sensor. (actually it is the low pass filter in front of the sensor) Do you see any dust particles? If so, use a bulb blower (or if you must, a Giotto Super Rocket Hurricane blow the toupee off your neighbors head from 60ft. blower) to blow the dust off and out. It may take several tries but if you can no longer see the dust, chances are it's no longer there. You may have to try this a couple of times to get it all off the low pass filter. If it is, and it's not moving around, you will need professional tack type cleaning. The dust may be welded in place.
2) If that blower method is successful, chances are you now have dust and crap (like hair) on the focus screen which is now driving you crazy because your beautiful clear matte viewfinder has specks. This won't appear on your images but is very annoying. In trying to blow the focus screen off you will inevitably blow dust into the chamber between the focus screen and viewfinder lens and possibly back on the sensor. It is now impossible to remove this dust from the backside of the focus screen without having to remove it. So, if you can live with a little dust on your focus screen and your pictures are clean you've got it. Otherwise, take it to the proffesional. Removing and installing the focus screen is not for the faint of heart and if you scatch it there is no user installable replacement for the K100D other than after market split focus screens. If your using a K10 you can get another from Pentax for a price.
3) Always change lenses with the body opening facing down. I've seen camera store clerks lay the body, lens off, opening up, on counters while changing lens on a DSLR in the store. All I could do is shutter. No pun intended. Inspect the rear of the lens to make sure the lens your installing is clean and try to be as expeditious as possible when swapping your lenses. Always protect that body opening. If there's wind blowing or a fan running try to get away from that. Hold the camera down inside your camera bag and change lenses there if at all possible.
4) If you use Photoshop and for some reason you have dust spots on your pictures like the one above, zap em with the healing brush tool. Piece O' Cake.
Just remember, a little caution when changing lenses and a little puff from a bulb blower from time to time will keep you from plopping down extra LBA dollars for sensor cleaning at the local shop.
And believe me, I've been through it all and survived.
Good Luck
Chuck