  The ttbar spin correlation at production is a fundamental prediction of QCD
and a potentially incisive test of new physics coupled to top quarks. We
measure the ttbar spin state in ppbar collisions at sqrt(s) = 1.96 TeV using
1001 candidate events in the lepton plus jets decay channel reconstructed in
the CDF II detector. In the helicity basis, for a top-quark mass of 172.5
GeV/c^2, we find a spin correlation coefficient kappa = 0.60 +/- 0.50 (stat)
+/- 0.16 (syst), consistent with the QCD prediction, kappa ~= 0.40.
