  We report a combination of the CDF and D0 measurements of the inclusive
single top quark production cross section in the s- and t-channels in ppbar
collisions at a center of mass energy of 1.96TeV. The total integrated
luminosity included in CDF's analysis is 3.2 fb^-1 and D0's analysis has 2.3
fb^-1. A Bayesian analysis is used to extract the cross section from the
distributions of multivariate discriminants provided by the collaborations. For
a top quark mass m_t=170 GeV/c^2, we measure a cross section of 2.76 +0.58
-0.47 pb. We extract the CKM matrix element |Vtb|=0.88+-0.07 with a 95% C.L.
lower limit of |Vtb|>0.77.
