There are two possible causes for a blurred subject: 1) the subject is out of focus, and 2) the subject and/or the camera is moving and the shutter speed is too slow to "stop" the motion.
For moving subjects, both 1) and 2) come into play.

You might think that modern autofocus lenses solve problem 1), but this isn't completely true, especially in poor lighting conditions. Not only does the autofocus system have trouble in low light, but your camera will typically try to shoot with the lens wide open, which minimizes your depth-of-field and thereby increases the chance that your subject will be out of focus. For better depth-of-field, stop down your lens (to a higher F-stop).

A moving subject or camera can be corrected by a faster shutter speed, but keep in mind that the longer your lens (or the higher your zoom) the faster your shutter needs to be. The best way to deal with the moving camera issue is to use a tripod. If this isn't practical, you might want to try an image-stabilized lens.

In sports photography, everything is working against you. Stopping down your lens and increasing your shutter speed both reduce the amount of light hitting your sensor. To correct for this, you need to increase your ISO, which makes your picture grainier. Moral: the best sports photographs are usually taken in bright sunlight.

I've been shooting with the k5 for several months now and I'm very happy with it. It's really best for stills -- for serious video work I would go with the Canon 7D. For a high-end DSLR, it's pretty small and lightweight, but the build quality is quite good. I've had great results with high ISO exposures -- 800 has little or no noticeable noise, and I've gotten pretty amazing results at 1600 and 3200. I've even shot some OK-looking snapshots at 6400. The in-camera HDR feature is fun, and you can get a decent shot in previously impossible lighting conditions. Of course you can do this in post, but it's cool to be able to see a result right away.

The main reason I got the K5 is that I already had several Pentax lenses. But one disadvantage of Pentax is that the selection of high-end lenses is nowhere near as good as you can get with, say, Canon. Especially if you're into super-telephoto.