Barnster, I'll try to sidestep the battle of the brand defenses and simply share that I own both cameras. I've had the D90 for longer than the K-7 so I would say I'm even more familiar with it. I currently own the 18-105mm VR and 70-300mm VR for the Nikon and I own three Limited lenses (21, 35, and 70) along with a couple of manual focus primes (28 and 50) for the K-7.
The bottom line is that these are two fairly different setups with the two systems and I use them in totally different ways. That said, I find myself reaching for the K-7 more than the D90. The D90 is fast, responsive, has great ISO performance, and takes really awesome photos. The K-7 has amazing build quality, incredible lenses, and a beautiful design that makes it a pleasure to shoot with.
Each has their strengths and weaknesses but the real differences are in the lenses available for each. If you're a consumer zoom shooter and have no plans on changing that, (let the booing begin) the D90 has an amazing 18-105mm VR that is simple a great lens. If you like more compact primes and are looking for a more refined feel with the body and lenses, however, it's going to leave you a bit frustrated. Nikon produces cheap, plastic consumer lenses nowadays. Pentax has some dogs as well but their Limiteds are simply unmatched. If you have no interest in Limiteds or the DA* weather-sealing, Pentax simply might not be the choice for you.
I would encourage you to actually handle each if at all possible. The build quality of the K-7 is on par with the Nikon D300 and leaves the D90 seriously behind in terms of quality of feel. Add in some of the higher end Nikon glass and you're looking at a serious credit card bill. My advice is to define 1) what kind of lenses are you going to buy, not what you'd like to be able to afford, but what you will actually spend money on, 2) determine what type of photography you actually do, do you need extremely fast autofocus and high ISO or do you, like most everyone, take most of your photos at ISO 800 or lower?, and 3) get a chance to handle each camera and take some test photos. This will settle the deal faster than any other option.
So, the bottom line is that even though I own both systems, I take the K-7 with my 90% of the time. The only reason I haven't sold the D90 yet is that I still use the 70-300mm VR for wildlife until I can figure out if the Sigma 100-300mm f/4 I purchased can be fixed for my camera. Once I can sort out the wildlife lens option, the Nikon may go bye bye -- PM me if you're just dying to purchase a used Nikon D90 and two lenses!