  We search for new charmless decays of neutral b-hadrons to pairs of charged
hadrons with the upgraded Collider Detector at the Fermilab Tevatron. Using a
data sample corresponding to 1 fb-1 of integrated luminosity, we report the
first observation of the B0s->K-pi+ decay, with a significance of 8.2 sigma,
and measure BR(B0s->K-pi+)= (5.0+-0.7(stat)+-0.8(syst))*10^{-6}. We also report
the first observation of charmless b-baryon decays in the channels Lambda_b ->
p pi and Lambda_b -> pK with significances of 6.0 sigma and 11.5 sigma
respectively, and we measure BR(Lambda_b->p pi-) =
(3.5+-0.6(stat)+-0.9(syst))*10^{-6} and BR(Lambda_b->p K-) =
(5.6+-0.8(stat)+-1.5(syst))*10^{-6}. No evidence is found for the decays
B0->K+K- and B0s -> pi+pi-, and we set an improved upper limit BR(B0s ->
pi+pi-) < 1.2*10^{-6}$ at the 90% confidence level. All quoted branching
fractions are measured using BR(B0->K+pi-)$ as a reference.
