  Hydrogen atoms inside virialized minihalos generate a radiation background
from redshifted 21-cm line emission whose angular fluctuations reflect
clustering during before and during reionization. We have shown elsewhere that
this emission may be detectable with the planned Low Frequency Array (LOFAR)
and Square Kilometer Array (SKA) in a flat LambdaCDM. This is a direct probe of
structure during the ``Dark Ages'' at redshifts z>6 and down to smaller scales
than have previously been constrained. In our original calculation, we used a
standard approximation known as the ``linear bias'' [e.g. Mo & White (1996)].
Here we improve upon that treatment by considering the effect of nonlinear
clustering. To accomplish this, we develop a new analytical method for
calculating the nonlinear Eulerian bias of halos, which should be useful for
other applications as well. Predictions of this method are compared with the
results of LambdaCDM N-body simulations, showing significantly better agreement
than the standard linear bias approximation. When applied to the 21-cm
background from minihalos, our formalism predicts fluctuations that differ from
our original predictions by up to 30% at low frequencies and small scales.
However, within the range of frequencies and angular scales at which the signal
could be observable by LOFAR and SKA as currently planned, the differences are
small and our original predictions prove robust. Our results indicate that
while a smaller frequency bandwidth of observation leads to a higher signal
that is more sensitive to nonlinear effects, this effect is counteracted by the
lowered sensitivity of the radio arrays. We calculate the best frequency
bandwidth for these observations to be Delta nu~2 MHz.(abbreviated)

Astrophysics