  We use numerical hydrodynamical simulations of early structure formation in a
LCDM universe to investigate the spin temperature and 21cm brightness of the
diffuse intergalactic medium (IGM) prior to the epoch of cosmic reionization,
at z<20. In the absence of any radiative heating, collisions between neutral
hydrogen atoms can efficiently decouple the spin temperature from the CMB only
in dense minihalos and filaments. Shock heated gas shines in emission,
surrounded by cooler gas visible in absorption. In the case of a warm, mostly
neutral IGM, produced here by X-ray emission from an early miniquasar, the 21cm
signal is strongly enhanced. Even slightly overdense filaments now shine in
emission against the CMB, possibly allowing future radio arrays to probe the
distribution of neutral hydrogen before reionization.
