  We present interstellar scintillation velocity measurements for four
millisecond pulsars obtained from long-term monitoring observations with the
Arecibo radio telescope at 430 MHz. We also derive explicit expressions that
relate the measured scintillation velocity to the effective transverse velocity
responsible for the motion of the diffraction pattern for both binary and
solitary pulsars. For B1257+12, B1534+12, J1640+2224, and J1713+0747 we derive
velocity estimates of 197, 192, 38, and 82 km/s, respectively. These values are
in good agreement with proper motion measurements for the four pulsars. For PSR
B1534+12, we use the ISS velocity dependence on orbital phase to determine the
longitude of the ascending node (Omega) of the pulsar's orbit and to derive an
estimate of the effective scattering screen location. The two possible values
of Omega are 70+/-20 and 290+/-20 degrees and the approximate screen location
is 630+/-200 pc with the assumed pulsar distance of 1.1 kpc.
