  The amount and properties of high-redshift galactic and intergalactic (IG)
dust are largely unknown, but could be investigated using multi-wavelength
photometry of high-z objects that have a known intrinsic spectrum. Observations
of gamma-ray burst (GRB) afterglows appear to support the theoretical model of
an adiabatic blast wave expanding into an external medium. In this model, the
synchrotron peak flux is independent of frequency, providing a flat spectrum
when observed over time, and therefore allowing straightforward measurement of
the relative attenuation of afterglow flux in widely separated bands. Applying
this method to dust extinction, we show that for a sample of afterglows which
have been corrected by galactic extinction, comparison between the number
counts of peak fluxes in X-ray versus optical can provide constraints on an
intergalactic component of dust. A similar technique can probe the
redshift-dependence of extinction in GRB-forming regions without requiring an
assumed relation between extinction and reddening by the dust. Probing
systematic changes in extinction with redshift - particularly in IG and/or
non-reddening dust - is crucial to a proper interpretation of the Type Ia
Supernova Hubble diagram and similar observations, and useful in understanding
GRB progenitor environments.
