Steven's (volosong's) reply is very good. Let me put it just a bit differently.
If you're starting from scratch, buying your first digital slr (perhaps your first slr of any kind), then you have a lot of choices. As far as I can tell, almost every dslr on the market today can take really good photos. So in one sense, you can't go wrong.
But that doesn't mean that the shopping is easy.
I think the main reason to buy Pentax right now is economic. Pentax cameras, especially since the release of the K100D and the K10D, are excellent photographic tools. They are not better than the best that Nikon and Canon have to offer, not in any definitive, objective way. 10,000 pro photographers using Canon and Nikon can't be completely wrong. But with Pentax, you get bodies that compare to Canon and Nikon products that cost much more. The K100D is cheaper than the Canon XTi and a better camera in nearly every way. The K10D has been compared very favorably to the Nikon D200, which sells presently for twice as much. The K20D looks like it's going to compare favorably, at least in many important respects (chiefly, image quality) to the new Nikon D300, which costs at least 50% more.
The same thing is true of lenses. Pentax users boast about the quality of the Pentax lenses, especially the primes. It's a fair boast. But if the boast is meant to imply that there aren't any good lenses for Nikon or Canon cameras, well, then the Pentax boast is simply wrong. I don't know a lot about Canon, but Nikon makes some terrific lenses. And Zeiss makes even better lenses. The Zeiss k-mount primes appear to be superior even to the Pentax primes; and Zeiss makes the same lenses in a Nikon mount, too.
It's at this point that I mention that Pentax cameras now have shake reduction built into the bodies. Now I'm a fan of shake reduction, a.k.a. vibration reduction, a.k.a. image stabilization. But Canon and Nikon cameras have it, too--it's just built into their lenses, rather than into the body. As far as I can tell, in-body shake reduction is no better than lens-based image stabilization and it may be that Canon and Nikon are right in claiming that lens-based IS is slightly better. But in-body shake reduction is MUCH MORE ECONOMICAL.
So if we're just talking about quality here, then either it's a draw, or Canon and Nikon win. The Nikon D3 looks like it's the best digital SLR ever made, hands down. I don't own one for a very simple reason: because it costs $5000, well, and because after I shelled out five grand for the camera, I'd have to spend another five grand to come close to matching the Pentax system I've got. The Zeiss lenses appear to be technically superior even to the great Pentax primes, but I don't own any Zeiss lenses because I can't afford them. No, that's not quite it. I could afford at least one--after all, I own a Pentax DA* 50-135 f/2.8 and it cost more than my camera. But at some point, I ask myself whether the superiority of the Zeiss lens is really worth paying double for, and I answer no. For the price of one Zeiss lens (costing say $800) I can buy a pair of excellent Pentax primes, and even the experts will be hard pressed to tell the difference in the photos. And the same goes for the camera bodies. I think I would have been just about as happy with a Nikon D200 as I am with the Pentax K10D. Except that, if I'd bought the D200, I would have a much smaller collection of lenses to work with--or I'd have the same types of lenses, but I would have cheap lenses, because my budget is finite.
SO, a Pentax system gives you equipment that's competitive with what you have available in Nikon and Canon systems, equipment that can produce truly great, brilliant photographs. And for comparable gear, the Pentax photographer will pay much less than the Canon or Nikon photographer.
The K10D is a terrific camera. The ergonomics are terrific, it's a joy to use. Compared to the Pentax K10D, the Canon Rebel XTi feels cheap, and the Nikon D40 feels and handles like a toy camera. If you make your decisions based primarily on financial considerations, then that's all you need to know: For under $1000, you will do much better with Pentax. If on the other hand you're also thinking about getting the BEST system, then the choice is much harder, because if you're willing to pay for quality, you can do great with Pentax, but you can as well or better with Canon and Nikon.
Two final points. In my opinion, the single biggest weakness of the Pentax system is the P-TTL flash. If you want to be a wedding photographer, for example, Pentax might not be the best platform, because flash is really important for wedding and event photographers and Nikon's flash system is clearly better than Pentax's.
The other point is that, if you think you might become a pro, there's a lot to be said for going with Nikon or Canon. The vast majority of the world's pros use Canon and Nikon cameras most of the time (when they're not using their Hasselblads and Leafs and such). If you're going to go pro with Pentax equipment, you have to get used to NOT being able to buy a lens on the spur of the moment from the local camera store, NOT having priority service available for pros, NOT being able to rent bodies and lenses easily, and you must be willing to be a bit of a maverick among your pro friends, most of whom will know very little about Pentax.
