What is your lens lineup like?
The k-X is very inexpensive and has high ISO functionality rivaling the D90. It is not weather sealed / have as many AF points, but it is relatively inexpensive will be a way better deal than chasing the most advance available tech.
The reason I ask is because you are comparing 3 brands - which I assume means you don't have any money invested in lenses.
If you have money to blow and are looking for state of the art tech, look to Nikon (or Canon really). Those two companies have very robust R&D. Pentax is a value brand - great quality lenses, PRACTICAL bodies, and huge savings.
You can take the same picture with any system - if you are good enough. And the fancy tech of nikon/canon will not mask the work of a bad photographer.
I'm inclined to think that Pentax isn't interested in "topping that" - their business strategy seems to be collecting Pentax owners at the entry level to boost lens sales. To be perfectly honest, the aging k20d takes great pictures, and I'm beginning to doubt whether future improvements will be anything like that we have seen in camera technology over the past 10 years. At a certain point, more megapixels really just translates out into less free disk space, and high ISO performance is already reaching the point of night vision
.