Canon just doesn't resonate with me. I find the UI to not be that intuitive, and their good glass is really expensive. Plus they are known for long/fast lenses and shooting sports. That isn't really my bag. For video though, I am a total Canon fan. I love my XH-1A. And their SD point and shoots are about the best "consumer" camera out there imho. I have used an SD400 for a couple of years and gotten great shots out of it with never a problem.
Pentax is kind of a weird duck these days (so it fits me perfectly). Canon is the 800lb gorilla, Nikon is the 500lb gorilla, and everyone else is an also-ran. I think that those two make up about 94% of the dslr market. That being said, somewhat like autos, niche players can make a go if they have something going for them. Olympus created their own lens/sensor spec and made it "open". Lecia has supported it, although not sure if they will continue (that is another interesting corporate tale). They also have other products to help fund R&D.
Pentax's edge is that they are a camera/lens company. They don't do other stuff. But they don't have huge budgets so they have to be creative and limit their reach. One big selling point is that almost EVERY lens that Pentax has made over the past 60 or so years will fit the current dslrs. You won't get auto focus (and with the ones older than 25 years or so, you also won't get auto aperture), but they will mount up and they will function. Pentax historically made some excellent lenses, so for film guys going digital with a bunch of glass, it is a fairly easy choice.
Their bodies are a mixed bag. The new top-end one, the K20d, uses a Samsung sensor (they partnered with them, and likely are getting cash infusion from them as well) that is 14.6MP. That is best in class so far for an APC sensor. Pentax is behind the game though in auto focus performance and in fast shooting (frames per second). Nikon's new D300 has amazing AF and will go 6-8 fps. The K20d will top out at about 3fps. But the K20d has in-body stabilization which makes a difference. Nikon does the stabilization in the lenses. So with Nikon you pay more for a VR lens. With Pentax, you can mount a 40 year old lens and get image stabilization. Pretty cool.
But the Pentax bodies seem like they were designed by photographers instead of engineers. Controls are laid out logically and work like you'd expect them too. Better than Nikon imho, and vastly better than Canon. The Olympus E3 seems like it was designed by committee, and the buttons are made for very small fingers. But those are personal things.
If you go by specs, the K20d has more mp (which doesn't really matter these days) but gets outperformed in AF, FPS, and high ISO shooting. But it is also cheaper ($1229 vs. $1749 for the D300).
The biggest thing are the lenses though. Nikon is making some amazing zoom lenses. But I don't really like zoom lenses. Pentax has a series of "limited" prime lenses. The 77 is one of the FA series which are old-school auto focus lenses. The 31mm has god-like status in the photography world and the 77mm is also highly regarded. (
http://www.luminous-landscape.com/columns/sm-02-05-02.shtml). They also make "pancake" primes in 21mm, 40mm and 70mm that are small and light, metal construction, and perform really well.
The bottom line is that it is really hard to go wrong with any dslr made these days. It really depends on what you want to shoot, how you shoot, and what you bond with. Pentax is kind of a funky company with very cool glass and a very good performing system. It will lose in the spec wars, but I don't take pictures of brick walls and newspapers (a favorite of online camera forum guys who I swear never take pictures of anything other than "test" shots).