Abstract: Much of our understanding of dark matter halos comes from the largely-untested assumption that the mass-to-light ratio of spiral disks is constant is radius. To test this hypothesis, we have used planetary nebula kinematics to measure the disk mass surface density of five nearby, low-inclination spiral galaxies. We show that, in general, disks do have a constant M/L out to ~ 3 optical scale lengths, but once outside this radius, the vertical velocity dispersion of old disk stars is independent of distance. We suggest that this is due to the increased importance of a thick disk, and possible disk heating of the thin disk by dark halo substructure. We show that the disks of early type spirals have higher M/L values and are closer to maximal than the disks of later-type spirals, and that the residual unseen dark halos are better fit by pseudo-isothermal laws than by NFW profiles.