well for starters, what lenses do you own for the Pentax? If you have a collection brewing already, I would'nt switch over to a new system entirely, and if you want newer, then move up in the Pentax line (unless you only have like. the kit lens).
The Canon/Nikon line is nice, and has TONS of avaliable accessories, but for actual quality, other brands aren't all that different, you're paying for the name. Now Canon and Nikon seem to be best with quality a little bit, but not leaps and bounds so don't worry. I'd say the Pentax K10D is looking like a nice camera if you want to upgrade. Dust sealed and gasketted, self cleaning sensor, and built in image stabilization plus the use of your current lenses.