See:-
http://www.dxomark.com/index.php/eng/Image-Quality-Database/Pentax/Kx
As I predicted last time (in December 2009), the DxO Mark for the K-x would be very close to that of the Nikon D90 and D5000 and yet once again I am correct! :-)
Nevertheless, the K-x performs really well for an APS-C cropped DSLR. It is ranked at 19 for all DSLRs tested so far. Well, even it is not the best APS-C, it is already very close to the Nikons and actually the difference is almost negligible. (As for the K-7, the results are just pathetic, in contrast.)