  The 21cm forest -- HI absorption features in the spectra of high-redshift
radio sources -- can potentially provide a unique probe of the largely neutral
intergalactic medium (IGM) during the epoch of reionization. We present
simulations of the 21cm forest due to the large scale structure of the
reionization-era IGM, including a prescription for x-ray heating and the
percolation of photoionization bubbles. We show that, if detected with future
instruments such as the Square Kilometer Array (SKA), the 21cm forest can
provide a significant constraint on the thermal history of the IGM. Detection
will be aided by consideration of the sudden increase in signal variance at the
onset of 21cm absorption. If radio foregrounds and the intrinsic source spectra
are well understood, the flux decrement over wide bandwidths can also improve
detection prospects. Our analysis accounts for the possibility of narrow
absorption lines from intervening dense regions, but, unlike previous studies,
our results do not depend on their properties. Assuming x-ray heating
corresponding to a local stellar population, we estimate that a statistically
significant detection of 21cm absorption could be made by SKA in less than a
year of observing against a Cygnus A-type source at $z \sim 9$, as opposed to
nearly a decade for a significant detection of the detailed forest features. We
discuss observational challenges due to uncertainties regarding the abundance
of background sources and the strength of the 21cm absorption signal.
