  The broad X-ray iron line seen in the spectra of many AGN is thought to
originate from the inner regions of the putative black hole accretion disk, and
hence provides a rare probe of that central region. In principle, future high
throughput X-ray spectrometers should be able to examine the temporal response
of this fluorescent line to flares in the X-ray continuum source (which
energizes this emission line) - i.e. iron line reverberation mapping will be
possible. It has been previously found that there are robust reverberation
signatures of the black hole spin, mass and the X-ray flare location above the
accretion disk. Here, we simulate observations of a bright Seyfert nucleus with
the proposed NASA mission Constellation-X and demonstrate the feasibility of
detecting these reverberation signatures with this mission. Hence, starting
with XMM in 1999, and maturing with Constellation-X in c2010, iron line
reverberation will open a new window on the innermost regions of AGN.
