I've got the Pentax K10D, and it's a very good camera. I recommend it. I've been a Pentax user for many years, and I wanted a camera that would be compatible with the lenses that I already owned, so I didn't really investigate the Canon and Nikon offerings enough to be able to make a comparison. I'm sure they're pretty good too.
An interesting feature of the Pentax is the "shake reduction", which is built into the body and so works with any lens. It works really well. Pentax's lenses are also superb. I've seen pictures from the new range of DA* zooms, and they look great, although they may stretch your budget a bit. For fixed focal length lenses, any from the 'Limited' range are hard to fault. Even the entry level lenses, such as the 18-55, are better than you have any right to expect, for the price. That's not always the case with budget lenses from other manufacturers.
Melvil mentions issues with image sharpness. This is really nothing to worry about. It something that affects the JPEG images from the camera, and it's true that they can be marginally less sharp than JPEGs from other makers' cameras. Pentax have just been a bit conservative with the amount of in-camera sharpening that they do. It's easy to add a bit more on the computer if you need it, much harder to remove it when you've got too much. If you shoot RAW (and if you're bothered about such small differences in quality then you probably will) then you'll do the sharpening yourself on your computer anyway.
Perhaps you should also consider the Pentax K100D "Super". It's only six megapixels compared to the K10D's ten, but it has similar shake reduction, takes all the same lenses, etc. (I think only the new 'Super' model is compatible with the ultrasonic focusing feature on the new DA* lenses). Since it has fewer megapixels it has slightly better noise performance in low light, if that's important to you. And of course it's cheaper, so you can spend more on lenses.
It's probably clear that I'm a bit of a Pentax fan. However, I second Melvil's suggestion that you go and try them all out in a shop. Technically, they're all good. Pick the one that feels nice in your hands (consider an add-on grip if that feels better to you), and with a viewfinder that you like. Especially if you wear glasses - check that you can see the viewfinder clearly.
PZ.