  We investigate the potential of exploiting Lya Emitters (LAEs) to constrain
the volume-weighted mean neutral hydrogen fraction of the IGM, x_H, at high
redshifts (specifically z~9). We use "semi-numerical'' simulations to
efficiently generate density, velocity, and halo fields at z=9 in a 250 Mpc
box, resolving halos with masses M>2.2e8 solar masses. We construct ionization
fields corresponding to various values of x_H. With these, we generate LAE
luminosity functions and "counts-in-cell'' statistics. As in previous studies,
we find that LAEs begin to disappear rapidly when x_H > 0.5. Constraining
x_H(z=9) with luminosity functions is difficult due to the many uncertainties
inherent in the host halo mass <--> Lya luminosity mapping. However, using a
very conservative mapping, we show that the number densities derived using the
six z~9 LAEs recently discovered by Stark et al. (2007) imply x_H < 0.7. On a
more fundamental level, these LAE number densities, if genuine, require
substantial star formation in halos with M < 10^9 solar masses, making them
unique among the current sample of observed high-z objects. Furthermore,
reionization increases the apparent clustering of the observed LAEs. We show
that a ``counts-in-cell'' statistic is a powerful probe of this effect,
especially in the early stages of reionization. Specifically, we show that a
field of view (typical of upcoming IR instruments) containing LAEs has >10%
higher probability of containing more than one LAE in a x_H>0.5 universe than a
x_H=0 universe with the same overall number density. With this statistic, a
fully ionized universe can be robustly distinguished from one with x_H > 0.5
using a survey containing only ~ 20--100 galaxies.
