I have the K-7 and it is a wonderful DSLR. It's my 3rd Pentax DSLR and the K-7 has convinced me yet again that Pentax, although a much smaller maker than either Canon or Nikon, really know how to make a camera that has great features for the photographer and
just works! Their MZ-5n SLR of the late 1990s was the same. Not bigger or heavier than it needs to be, it is packed with customisable options so you can set up the controls the way that works for you and the image quality is right up there on par with the Nikon and Canon models that are one step more expensive. In body Shake Reduction is handy for at least 2-3 stops (sometimes more) slower shutter shooting and of course every lens benefits, not just the expensive in-lens IS that Nikon and Canon use.
Pentax also has the best range of jewel like "Limited" prime lenses if you want to get into some very nice gear. Play with a couple of these Limited lenses and you'll be drooling at the build and image quality and wonder why the other makers abandoned this part of the market. They aren't for everybody but if you appreciate the tactile pleasure of slowing down your photography and using high quality primes lenses (more discrete too for street photography) then you'll start getting more out of your hobby with this gear:
http://www.pbase.com/shreder/image/118249197.. and lastly, why run with the crowd? It's the other reason why I stick with Pentax (and switched from a PC to a Mac), I'd rather not run with the heard, it's a special thing to be a Pentaxian and do things just a little differently. Your
photos really won't be any different whether you choose a D90 or K-7 but what you get out of it might be a little different...
Either way, enjoy..