  We report measurements of the polarization of W bosons from top-quark decays
using 2.7 fb^-1 of ppbar collisions collected by the CDF II detector. Assuming
a top-quark mass of 175 GeV/c^2, three measurements are performed. A
simultaneous measurement of the fraction of longitudinal (f_0) and right-handed
(f_+) W bosons yields the model-independent results f_0 = 0.88 \pm 0.11 (stat)
\pm 0.06 (syst) and f_+ = -0.15 \pm 0.07 (stat) \pm 0.06 (syst) with a
correlation coefficient of -0.59. A measurement of f_0 (f_+) constraining f_+
(f_0) to its standard model value of 0.0 (0.7) yields f_0 = 0.70 \pm 0.07
(stat) \pm 0.04 (syst) (f_+ = -0.01 \pm 0.02 (stat) \pm 0.05 (syst)). All these
results are consistent with standard model expectations.
