  [Abridged] We report the discovery of the near-infrared and optical afterglow
of the short-duration gamma-ray burst GRB070724A. The afterglow is detected in
i,J,H,K observations starting 2.3 hr after the burst with K=19.59+/-0.16 mag
and i=23.79+/-0.07 mag, but is absent in images obtained 1.3 years later.
Fading is also detected in the K-band between 2.8 and 3.7 hr at a 4-sigma
significance level. The optical/near-IR spectral index, beta_{O,NIR}=-2, is
much redder than expected in the standard afterglow model, pointing to either
significant dust extinction, A_{V,host}~2 mag, or a non-afterglow origin for
the near-IR emission. The case for extinction is supported by a shallow optical
to X-ray spectral index, consistent with the definition for ``dark bursts'',
and a normal near-IR to X-ray spectral index. Moreover, a comparison to the
optical discovery magnitudes of all short GRBs with optical afterglows
indicates that the near-IR counterpart of GRB070724A is one of the brightest to
date, while its observed optical emission is one of the faintest. In the
context of a non-afterglow origin, the near-IR emission may be dominated by a
mini-supernova, leading to an estimated ejected mass of M~10^-4 Msun and a
radioactive energy release efficiency of f~0.005 (for v~0.3c). However, the
mini-SN model predicts a spectral peak in the UV rather than near-IR,
suggesting that this is either not the correct interpretation or that the
mini-SN models need to be revised. Finally, the afterglow coincides with a star
forming galaxy at z=0.457, previously identified as the host based on its
coincidence with the X-ray afterglow position (~2" radius). Our discovery of
the optical/near-IR afterglow makes this association secure.
