Bass - Great info from Dale; I'd go through the excercises he suggests before buying anything.
If a camera doesn't feel right in your hands...don't buy it! Have you played with a K10D?
I don't do much low light shooting, mainly nature/landscape and wildlife. That said, I've got a Pentax 31mm f/1.8 ltd. that's awesome for both sharpness and low light (see the review of it on the LL site). It's expensive but worth it. BTW, I really like the Pentax glass I have (I have a Sigma 50-500mm too - The "Bigma") but Pentax is behind Canon and Nikon in overall quantity of selections. With the in-camera IS, the glass you will buy is generally less expensive though. Having said that, you'll still need a tripod - the IS is good but not as good as the Pentax marketing department would have it!
The K10D ISO 1600 (based on my experience with it and having none with any other camera) isn't bad and cleans up well with Noise Ninja (the software I use, there are other good ones). After today's shoot, I am thrilled with the K20D at ISO 1600; it's like ISO 400 on the K10D, which is to say very good! I was shocked the ISO 3200 was as good as it was too. Their new CMOS 14.6mp resolution is a big upgrade in image quality, which is where it's at for me.
Still, I wouldn't hesitate to buy a K10D, (I kept mine as a back-up body) especially now that the K20D is out, as there should be some GREAT deals out there, with or without the 18-55mm kit lens. BTW, if you do buy one, go online and upgrade your firmwear to the 1.3 version.
I'm with Bass on the water issue - mist and a little light rain is no big deal with the Pentax weather seals but major spalshes like one might receive on a rafting trip would give me great pause. Water on the lens might be your biggest issue.
I hope this helps!
Phil