the pentax DSLR's main advantage is in-camera stabilization. this is perhaps more important to the entry-level user than the long-term hobbyist, enthusiast, semi-pro or pro. it effectively makes every lens you buy a VR/VC/OS/IS lens.
the downside to this is that you'll pay for that somewhere down the line if you want better glass. and what it comes down to isn't the bodies, but the glass.
for example, the tokina-made, pentax-branded 12-24, 16-50 and 50-135 cost considerably more than their equivalents in canon and nikon mount. and the other pro-spec pentax lenses are pretty pricey, too. on the plus side, pentax has some nice pancake lenses, a good line of f/4 zooms and primes, and there are plentiful legacy lenses available used which will fit a Pentax DSLR.
so, if you're either an entry-level person who doesnt fancy going too far past kit lenses, a shooter who likes primes exclusively, or someone willing to comb through ebay for used glass, the pentax system could work. the bodies are well-spec'd for the price, that's for sure. but specs are on thing on paper and another thing in the field. overall, canon and nikon have much more lenses. and nikon has the best flash metering system, period, as well as better ergonomics.
while the specs might seem similar, the d90 is clearly superior in one critical area: low-light performance. it also has better dynamic range and AF performance. in fact, the cheaper Pentax k-x is better at high-ISOs than the k-7 (according to dpreview).
that may or may not matter, however, depending on your style of shooting. if you do a lot of landscapes and posed portraits, the K-7 will be pretty good at base ISO in terms of resolution. but for sports, action or low-light, the K-7 might lag behind similarly-priced Canon or Nikon models.
for $100 difference, i'd probably get a d90, and if i really wanted to save money, i'd get a pentax k-x, but that's just me. if you're seriously considering pentax, i'd really look into the pentax lenses and really think about whether their selection/pricing appeals to you. it's a big decision, because you're not just buying a camera, you're buying into a system.
if you're familiar with nikon already, that's a point in their favor. if you only have one or two lenses, though, switching wont be that painful. in any event, you should take a hands-on test drive of the K-7 to see if you like their controls and feel. ditto with the d90.