I have an older Pentax, a K100D. Comparing them side-by-side with Canon and Nikon of the day, Pentax was way ahead in build quality and optical quality. (for the kit lens)
I agree they make a narrower selection of lenses, and the aftermarket also makes a narrower selection of lenses. But to me, it doesn't matter. The lenses I need, they have. They even have enough of a selection that they have some that I want that I don't really need. (the pancake lenses, specifically the 21mm f/3.2 and the wide angle zoom)
The body-integral anti-shake does what it says it does; it gives this benefit to ALL of my lenses, even my 50mm f/2 manual focus.
But Tyler is right int that it doesn't work quite as well as the optical systems that Canon has. It only gives me two stops more of hand-held steadiness. (that's enough for me) The optical image stabilization gives 3 stops, from what I've read. To be honest, it matters less these days, because the sensors in today's cameras are better. They can be shot at higher ISOs without a lot of degradation. On my old K100D, it looks great up to 800 ISO and the noise starts to become noticeable at 1600. I think today's SLR bodies can be shot up to 1600 or 3200 and still be great.
In your situation, I would resist the urge to update the body, and instead spend the cash on a quality lens. Get something fast if you're going to shoot action, and if not, get something stabilized. The kit lenses are really quite good for the price. But you don't really get all you can get out of an SLR until you put some really good optics on the body. Cheap body or expensive, it doesn't matter.
Remember that just a sprinkle of rain here and there is not going to ruin anything. Just don't let it get saturated.
I love my Pentax, but I think if I already had good Canon bodies and optics, I wouldn't start over with Pentax.
