I understand that prions have been implicated in the passing on of epigentic information. Are prions thought to play a significant role in the evolution of organisms?
It is proposed that prions are a good mechanism for "testing" phenotypic variation.
There are many identified proteins with prion-determining domains (PrD) in the yeast genome that can spontaneously switch between conformations with some low probability (eg: check SUP35 for one example, and  for a good overview of more). The theory is that:
Check out the excellent paper published just last week in Nature exploring this this topic . To give a sense of just how evolutionarily-advantageous prions can be, in the author's experiments and analysis they note that 40% of the prion traits they analyzed were beneficial to growth (eg: in the paper strain UCD939 gains additional resistance to acidic conditions from the prion [PSI+]).
Assuming these hypothesis, prions would thus play a significant role in the evolution and variability of organisms.