  We present early-time optical through infrared photometry of the bright
gamma-ray burst GRB 080607, starting only 6 s following the initial trigger in
the rest frame. Complemented by our previously published spectroscopy, this
high-quality photometric dataset allows us to solve for the extinction
properties of the redshift 3.036 sightline, giving perhaps the most detailed
information on the ultraviolet continuum absorption properties of any sightline
outside our Local Group to date. The extinction properties are not adequately
modeled by any ordinary extinction template (including the average Milky Way,
Large Magellanic Cloud, and Small Magellanic Cloud curves), partially because
the 2175-Angstrom feature (while present) is weaker by about a factor of two
than when seen under similar circumstances locally. However, the spectral
energy distribution is exquisitely fitted by the more general Fitzpatrick &
Massa (1990) parameterization of Local-Group extinction, putting it in the same
family as some peculiar Milky Way extinction curves. After correcting for this
(considerable, A_V = 3.3 +/- 0.4 mag) extinction, GRB 080607 is revealed to
have been among the most optically luminous events ever observed, comparable to
the naked-eye burst GRB 080319B. Its early peak time (t_rest < 6 s) indicates a
high initial Lorentz factor (Gamma > 600), while the extreme luminosity may be
explained in part by a large circumburst density. Only because of its early
high luminosity could the afterglow of GRB 080607 be studied in such detail in
spite of the large attenuation and great distance, making this burst an
excellent prototype for the understanding of other highly obscured
extragalactic objects, and of the class of "dark" GRBs in particular.

Cosmology and Nongalactic Astrophysics;High Energy Astrophysical Phenomena