  One of the key goals in observational cosmology over the next few years will
be to establish the redshift distribution of the recently-discovered sub-mm
source population. In this brief review I discuss and summarize the redshift
information which has been gleaned to date for the ~ 50 bright sub-mm sources
which have been uncovered via the six main classes of survey performed with
SCUBA on the JCMT over the last 2-3 years. Despite the biases inherent in some
of these surveys, and the crudeness of the redshift information available in
others, I conclude that all current information suggests that only 10-15 % of
luminous sub-mm sources lie at z < 2, and that the median redshift of this
population is z ~ 3. I suggest that such a high median redshift is arguably not
unexpected given current theories designed to explain the correlation between
black-hole mass and spheroid mass found at low redshift. In such scenarios,
peak AGN emission is expected to correspond to, or even to cause termination of
major star-formation activity in the host spheroid. In contrast, maximum dust
emission is expected to occur roughly half-way through the star-formation
process. Given that optical emission from bright quasars peaks at z = 2.5,
dust-emission from massive ellipticals might be reasonably expected to peak at
some point in the preceding ~ 1 Gyr, at z ~ 3. Confirmation or refutation of
this picture requires significantly-improved redshift information on bright
samples of SCUBA sources.
