  Pulsar timing measurements can be used to detect gravitational radiation from
massive black hole binaries. The ~106d quasi-periodic flux variations in
Sagittarius A* at radio wavelengths reported by Zhao, Bower, & Goss (2001) may
be due to binarity of the massive black hole that is presumed to be responsible
for the radio emission. A 106d equal-mass binary black hole is unlikely based
on its short inspiral lifetime and other arguments. Nevertheless the reported
quasi-periodicity has led us to consider whether the long-wavelength
gravitational waves from a conjectured binary might be detected in present or
future precision timing of millisecond pulsars. While present timing cannot
reach the level expected for an equal-mass binary, we estimate that future
efforts could. This inquiry has led us to further consider the detection of
binarity in the massive black holes now being found in nearby galaxies. For
orbital periods of ~2000d where the pulsar timing measurements are most
precise, we place upper limits on the mass ratio of binaries as small as 0.06.
