I've written up some brief notes on controlling shutter speed on the K-x: Pentax K-x - exposure (http://wildlifehealth.tennessee.edu/video/K-x.html)

One thing I found is that the K-x's ISO while in video mode appears to be 100 ... whereas in still mode its default minimum is 200. So if you select an aperture/shutter speed combination in Still image mode and then specify that aperture in Video mode, you'll find the camera using a shutter speed twice as long as you were expecting.

EDIT: It's been pointed out to be that the K-x's ISO can indeed be set to 100, allowing you to directly monitor the shutter speed that will be used in video mode. To access ISO 100, make sure the camera's "D-range setting" is not active, and that "Expanded Sensitivity" is selected.

Thank you Graham
It`s very complicated to aply your method.
Maybe one solution to try is from a Pentax Kx owner buddy, as myself (recently):
"ISO CONTROL: you go to AV mode, set the aperture to wide open, then pointing a light source, an by selecting an ISO of 400-800 or sth like that, you watch the shutter. If you get 1/25 (for us moviemakers 1/50) by changing the camera position towards the light source, you can got to video mode and lock the exposure. Then you have no more noise than ISO 400-800-noise - depending on what you had selcted.

Thanks Ron.

Perhaps I'm not understanding but it seems you and I are using variations on the same basic method: i.e., go to Av, pick an aperture/light combination that produces an acceptable shutter and ISO, then flick to Video mode to lock everything down.

Yes, I gess so. And lock / ajust exposure.

There have been some new discoveries on this topic - see the update here: Pentax K-x - exposure (http://wildlifehealth.tennessee.edu/video/K-x.html)

WARNING: OLD CRANK RANT MODE ON!
Why is it so hard for DSLR manufacturers to make a full manual mode, where EVERYTHING (ISO, aperture, shutter speed, focus) is set manually?

They didn't used to have this problem with film SLRs. My old Pentax K1000 35mm SLR doesn't even have any automatic mode.. at all, NADA, you set everything by hand. Pentax sold millions of 'em back in the day... even against fully automatic competition. Is there really no market for a fully manual camera? Did photographers just get stupider when they switched to digital?

RANT MODE OFF.

Regarding companies that sell video prosumer / professional cameras, I can figure out - they are supposedly protecting their investment.
But for Pentax there are no excuses, it`s a dumb attitude.
Other fault I can`t understand / accept is not include as standard a swivel LCD.
For example in Canon 5dMKII / 7d - It`s so simple and important...