We analyze the propagation of light in the context of nonlinear
electrodynamics, as it occurs in modified QED vacua. We show that the
corresponding characteristic equation can be described in terms of a
modification of the effective geometry of the underlying spacetime structure.
We present the general form for this effective geometry and exhibit some new
consequences that result from such approach.Comment: LaTex, 11 pages, accepted for publication in Phys. Rev.