The structure of the tbW vertex can be probed by measuring the
polarization of the W in t -> W + b -> l + v + b.  The invariant mass of the
lepton and b quark measures the W decay angle which in turn allows a
comparison with polarizations expected from a V-A and V+A  tbW vertex.  We
measure the fraction by rate of Ws produced with a V+A coupling in lieu of
the Standard Model V-A to be f_V+A = -0.21^+0.42_-0.24 (stat) +/- 0.21
(sys).  We assign a limit of f_V+A < 0.80 @ 95% CL.  By combining this
result with a complementary observable in the same data, we assign a limit
of f_V+A < 0.61 @ 95% CL.  From this CDF Run I preliminary result, we find
no evidence for a non-standard Model tbW vertex.