I don't know much about the Pentax *ist; heck, I only though it came in digital. But Pentax makes good cameras. When Nikon and Canon went to plastic lens mounts 10 years ago to save a nickle, Pentax stuck with metal lens mounts. But by now they've probably jumped on the "screw the consumer" band wagon just like the industry leaders (Nikon & Canon).
The Canon Rebel 2000 is a pretty good entry level SLR; I assume that other Canon Rebel models are also pretty good.
The Nikon N-80 is a few steps up from entry level. If I were ever to buy another auto focus 35mm SLR (I never will, I prefer less bells and whistles features, not more) it would probably be a Nikon N-80. It's a very nice camera.
In the long run the Canon and Nikon gear will have more accessories and lens choices. But Pentax will still have 99.9% of everything most photogs ever actually use. Pentax gear will probably be cheaper than Nikon or Canon gear.
Most of what is needed for good photography is in the photographer, and camera brand doesn't really matter. In fact, as a photog who uses many different designs of camera, I'd say there is hardly any difference between consumer model AF 35mm SLRs from manufacturer to manufacturer, but I know there are plenty of folks who swear by one brand name or another. Any of the major brands usually make pretty good equipment. Just make sure it has all the features you're going to want. Nikon, for instance, is notorious for leaving some pretty basic features off of their entry level SLRs (cable release, manual ISO selection, etc...).
I also have a very wierd attachment to my dads old Olympus OM1 thats kinda cool too.
