As the flow propagates, the density of the plasma (electrons, protons, pairs) decreases. Thus, above a certain radius, the optical depth becomes smaller than unity, and the photons escape. The radius at which this happens is, by definition, the photosphere. The first difficulty that arises when trying to calculate the properties of the photosphere, is that in GRBs the flow is highly relativistic, that is it travels at speeds very close to the speed of light, hence special relativity effects take place. As a result, the photospheric radius strongly depends on the angle to the line of sight. When I carried these calculations, I found that the dependence can be put in a surprisingly simple form: Here, is the Lorentz factor of the GRB relativistic outflow, and is the angle to the line of sight. The angular dependence of the photospheric radius appears in the figure below (note the logarithmic scale) 
Propagation of photons is a random process, and therefore not all of the photons decouple from the plasma exactly at the photospheric radius. In fact, photons have a finite probability of being emitted (=last scattering event) from every point in space. While the photosphere gives a first order approximation for the place where this emission takes place, a full calculation (see Pe'er 08) of this probability is required.
Figure 2, to the right, shows the results of a numerical simulation
that traces photons from deep in the flow until their escape. Every
point represent the last scattering position in the r 
plane (same as in figure 1). The
photospheric radius, drawn in green, indeed shows a first order
approximation to the last scattering event positions, which span the
entire space.

Since thermal photons originate from the photosphere, the innermost radius from which information can reach the observer, studying it may thus give important information on the progenitor, and the outflow. Thus, we believe that the results here are an important step towards understanding basic properties of GRBs. Below is a connection to further explanations on the implications of these results. The paper by Pe'er (2008) where the full theory is presented, can be found here. 
Study of thermal emission in GRBs: I identification 
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Study of thermal emission in GRBs: III Implications 