Playing with my new £5 bargain Tokina SD 28-70mm on the K10D tonight, I inadvertently rolled the wrong dial and change the ISO to 800. Thus far with my K10D (I've had it a couple of months) I've stuck to ISO100 99% of the time.
Far from being horribly unusable though, in my view the photograph below is more than acceptable and I actually quite like the noise of the background. It's far more like film grain than I've seen with many other digital cameras.
I don't plan to shoot often at ISO800, and will still shoot the vast majority of the time at ISO100 with the K10D, but this unexpected experiment has certainly reassured me I needn't worry too much about getting unusable images at ISO200, 400 or even 800 if/when the need arises!
Click the pic to see larger on Flickr (though the images I upload to Flickr are usually 50% size JPEGs of the original RAW file).