About 5 years ago, I got back into photography by going digital and essentially had the same set of choices you are currently facing. Buying into a system is true, however that is not right off the bat. An entry level camera with the single or dual kit lens set is really not locking yourself in. The 2 lens kit (18-55 and either the 55-200 or 55-300) lens essentially covers everything and are readily saleable, if you choose to switch. Its the next set of lenses that essentially pulls you in.
Pentax does have some limitations - in the telephoto area, i.e., going to 400, 500 or 600mm. If that is your interest then its either third party lenses or older Pentax models (and they are wonderful glass), however - you are talking about lots of money regardless of what system you are using.
Pentax tends to have very ergonomically engineered cameras. Its in body stabilization system makes what ever lens mounted stabilized. Canon and Nikon are lens stabilized systems thus you need to buy their stabilized lenses.
The new K5 is the equal of Nikon or Canon at the high end. As mtngal wrote, if you need a full frame - Pentax is not the brand for you as I doubt that they will ever go full frame. However for a well built, APS-c sensor based camera, its the equal of the rest.
The Kx is the equal of the entry level end. What ever you get, will be superseded by someones new model within 6 months time. It does not mean that it will not take great pictures - it just means what ever camera you decide upon, start taking pictures and stop looking at bodies or you will get on the endless upgrade cycle.
There is a very good - actually wonderful Pentax group here. The images are stunning. Everyone is friendly and more than willing to help a beginner - since we were all there at one time or another. There is also another extremely active group at pentaxforums.com
Pentax is alive and kicking and I do not believe going away any time soon.
Here are a few galleries with Pentax input - and I did not look too hard....