  Motivated by the recent detection by WMAP of a large optical depth to Thomson
scattering -- implying a very early reionization epoch -- we assess a scenario
where the universe was reionized by `miniquasars' powered by intermediate-mass
black holes (IMBHs), the remnants of the first generation of massive stars.
Pregalactic IMBHs form within minihalos above the cosmological Jeans mass
collapsing at z=24, get incorporated through mergers into larger and larger
systems, sink to the center owing to dynamical friction, and accrete cold
material. The merger history of dark halos and associated IMBHs is followed by
Monte Carlo realizations of the merger hierarchy in a LCDM cosmology. While
seed IMBHs that are as rare as the 3.5-sigma peaks of the primordial density
field evolve largely in isolation, a significant number of black hole binary
systems will form if IMBHs populate the more numerous 3-sigma peaks instead. In
the case of rapid binary coalescence a fraction of IMBHs will be displaced from
galaxy centers and ejected into the IGM by the `gravitational rocket' effect,
rather than accrete and shine as miniquasars. We show that, under a number of
plausible assumptions for the amount of gas accreted onto IMBHs and their
emission spectrum, miniquasars powered by IMBHs may be responsible for
cosmological reionization at z~15. Reionization by miniquasars with a hard
spectrum may be more `economical' than stellar reionization, as soft X-rays
escape more easily from the dense sites of star formation and travel further
than EUV radiation. Energetic photons will make the diffuse IGM warm and weakly
ionized prior to the epoch of reionization breakthrough, set an entropy floor,
and reduce gas clumping. Future 21 cm observations may detect a preheated,
weakly-ionized IGM in emission against the CMB. (abridged)
