  We discuss the effect of dispersion delay due to intervening ionized media in
the radio emission of gamma-ray bursts (GRBs). For high redshift GRBs ($z \ga
3$), the ionized intergalactic medium (IGM) should be the dominant source of
dispersion without substantial local or foreground contamination, offering a
unique probe of the cosmic reionization epoch through measures of the free
electron column density out to different redshifts. The delay times from $z
\sim 10$ can be $\sim$ 1 hour at 100 MHz and $\sim$ 10 hours at 30 MHz. On the
other hand, dispersion by local ionized material may be important for GRBs at
lower redshifts if they occur inside or behind dense molecular clouds,
providing clues to the GRB environment; free-free absorption may also be
significant in this case. Detecting dispersion delay in the known radio
afterglow emission should be extremely challenging due to the low fluxes at the
relevant frequencies, but may be marginally possible for rare, bright
afterglows by the {\it Square Kilometer Array}. If GRBs also emit prompt,
coherent radio emission as predicted by Sagiv & Waxman (2002), the
observational prospects can be much better; detection of a sufficient sample of
high $z$ GRBs by the {\it Low Frequency Array} may allow a discrimination of
different reionization histories. Interesting constraints on the low redshift,
warm-hot IGM may also be obtainable through dispersion.
