I've decided to git the Pentax K-r.
The K-x is my current go-to favorite. It is a tremendous camera for the size and price. And the K-r has all the K-x has, but souped up. A bit higher ISO settings, a bit stronger, a bit faster. In short, a bit more professional.
I admit I rarely need the pretty astounding 6 frames per second this one takes (the Canon 5D Mark II only does 3), but I think that speed is indicative of ruggedness and also of responsiveness, and when I have used a speedy camera (like the Nikon D200), I like it. It feels more trustworthy and at the same time feels more like a living thing working with you.
If you have a K-x, stick with it, but if you're considering a Pentax camera, I think the K-r or the fully pro K-5 are excellent options. Compared to Canon and Nikon, they have in-body stabilization which works with any lenses, including the many outstanding compact prime (non-zoom) lenses Pentax has (which is a weakness for the otherwise strong Nikon and Canon systems).
If you prefer working with a standard zoom, they now have built-in stabilization, and then Canon or Nikon will do you fine. They are all very good these days. (Note, Canon and Nikon also have some outstanding prime lenses, but they tend to be very big and heavy, and very expensive.)