Search for B-+/-->[K--/+pi(+/-)](D)K-+/- and upper limit on the b -> u amplitude in B-+/--> DK +/-

We search for B+/--->[K(-/+)pi(+/-)](D)K+/- decays, where [K(-/+)pi(+/-)](D) indicates that the K(-/+)pi(+/-) pair originates from the decay of a D-0 or (D) over bar (0). Results are based on 120x10(6) Y(4S)-->BB<top></top> decays collected with the BABAR detector at SLAC. We set an upper limit on the ratio R-Kpi=[Gamma(B+-->[K(-)pi(+)](D)K+)+Gamma(B--->[K(+)pi(-)](D)K-)][Gamma(B+-->[K(+)pi(-)](D)K+)+Gamma(B--->[K(-)pi(+)](D)K-)]</den></fraction><0.026 (90% C.L.). This constrains the amplitude ratio r(B)equivalent toparallel toA(B--->(DK-)-K-0)/A(B--->(DK-)-K-0)parallel to<0.22 (90% C.L.), consistent with expectations. The small value of r(B) favored by our analysis suggests that the determination of the Cabibbo-Kobayashi-Maskawa phase gamma from B-->DK will be difficult.