  Observations of QSOs at z ~ 5.7 - 6.4 show the appearance of Gunn-Peterson
troughs around z ~ 6, and a change in the slope of the IGM optical depth tau(z)
near z ~ 5.5. These results are interpreted as a signature of the end of the
reionization era, which probably started at considerably higher redshifts.
However, there also appears to be a substantial cosmic variance in the
transmission of the IGM, both along some lines of sight, and among different
lines of sight, in this intriguing redshift regime. We suggest that this is
indicative of a spatially uneven reionization, possibly caused by the
bias-driven primordial clustering of the reionization sources. There is also
some independent evidence for a strong clustering of QSOs at z ~ 4 - 5 and
galaxies around them, supporting the idea of the strong biasing of the first
luminous sources at these redshifts. Larger samples of high-z QSOs are needed
in order to provide improved, statistically significant constraints for the
models of these phenomena. We expect that the Palomar-Quest (PQ) survey will
soon provide a new set of QSOs to be used as cosmological probes in this
redshift regime.
