Well I think, that the K20D is more a concurrent of the be introduced D90 and maybe also a bit the Canon 40D but the 40D is a camera in an other class.
The K10D were also not a 1-1 comparable with the 30D.
Canon's 30D and 40D have a magnesium body more rugged faster AF(even the 30D) much faster FPS(even the 30D, let's don't talk about the 40D).
The Nikon D300 isn't really a concurrent for the K20D, the D300 is a camera with a much more robust body, a much better AF system. The max shutter speed of course twice as much, and flash sync also much higher. FPS speed is also much higher even without the extra battery grip(which is more advanced as the K10D/K20D grip with more buttons more place for batteries), let alone with the extra battery grip.
So no...the K20D is not really a worry for 'Nikon and 'Canon sales of their D300 and 40D.
But the K20D will be a success story for Pentax, I hope so. Not everybody needs a better AF system and more FPS and a more robust body and a higher flash sync speed
Many of the shooters just need LOW noise on high ISO's and a very good IQ
But we don't know if that's the case without full size RAW files