  We report a dramatic orbital modulation in the scintillation timescale of the
relativistic binary pulsar J1141--6545 that both confirms the validity of the
scintillation speed methodology and enables us to derive important physical
parameters. We have determined the space velocity, the orbital inclination and
even the longitude of periastron of the binary system, which we find to be in
good agreement with that obtained from pulse timing measurements. Our data
permit two equally-significant physical interpretations of the system. The
system is either an edge-on binary with a high space velocity ($\sim 115$ km
s$^{-1}$) or is more face-on with a much slower velocity ($\sim 45$ km
s$^{-1}$). We favor the former, as it is more consistent with pulse timing and
the distribution of known neutron star masses. Under this assumption, the
runaway velocity of 115 km s$^{-1}$ is much greater than is expected if pulsars
do not receive a natal kick at birth. The derived inclination of the binary
system is (76\pm 2.5^{\circ}) degrees, implying a companion mass of 1.01 (\pm
)~0.02 M(_{\odot}) and a pulsar mass of 1.29 (\pm)~0.02 M(_{\odot}). Our
derived physical parameters indicate that this pulsar should prove to be an
excellent laboratory for tests of gravitational wave emission.
