  We investigate the 21-cm signature that may arise from the intergalactic
medium (IGM) prior to the epoch of full reionization (z>5). In scenarios in
which the IGM is reionized by discrete sources of photoionizing radiation, the
neutral gas which has not yet been engulfed by an H II region may easily be
preheated to temperatures well above that of the cosmic background radiation
(CBR), rendering the IGM invisible in absorption against the CBR. We identify
three possible preheating mechanisms: (1) photoelectric heating by soft X-rays
from QSOs, (2) photoelectric heating by soft X-rays from early galactic halos,
and (3) resonant scattering of the continuum UV radiation from an early
generation of stars. In the presence of a sufficiently strong ambient flux of
Lyman-alpha photons, the hyperfine transition in the warmed H I will be
excited. A beam differencing experiment would detect a patchwork of emission,
both in frequency and in angle across the sky. This patchwork could serve as a
valuable tool for understanding the epoch, nature, and sources of the
reionization of the universe, and their implications for cosmology. We
demonstrate that isolated QSOs will produce detectable signals at meter
wavelengths within their ``spheres of influence'' over which they warm the IGM.
As a result of the redshifted 21-cm radiation emitted by warm H I bubbles, the
spectrum of the radio extragalactic background will display frequency structure
with velocity widths up to 10,000 km/s. Broad beam observations would reveal
corresponding angular fluctuations in the sky intensity with (delta T/T)=1E-3
on degree-scales. Radio measurements near 235 and 150 MHz, as will be possible
in the near future using the Giant Metrewave Radio Telescope, may provide the
first detection of a neutral IGM at 5<z<10.
