  Cold atomic gases resonantly excited to Rydberg states can exhibit strong
optical nonlinearity at the single photon level. We observe that in such
samples radiation trapping leads to an additional mechanism for Rydberg
excitation. Conversely we demonstrate that Rydberg excitation provides a novel
in situ probe of the spectral, statistical, temporal and spatial properties of
the trapped re-scattered light. We also show that absorption can lead to an
excitation saturation that mimics the Rydberg blockade effect. Collective
effects due to multiple scattering may co-exist with co-operative effects due
to long-range interactions between the Rydberg atoms, adding a new dimension to
quantum optics experiments with cold Rydberg gases.
