Do sub-mm sources and quasars form an evolutionary sequence?

The high redshift sub-mm sources discovered by SCUBA are widely believed to represent the dust-enshrouded formation of massive elliptical galaxies. Given the strong evidence for a link between the formation of the spheroid and the growth of the central black hole, one might expect to see a luminous quasar at the nucleus of every SCUBA source. Somewhat surprisingly, however, only a very small fraction (similar to 5%) are detected in X-rays, In this paper I discuss some of the implications of these results and discuss the accumulating evidence that sub-mm sources and quasars may represent different stages in the evolutionary sequence of a massive proto-spheroid.