Are prions an important driver in evolution?
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 [1] for a good overview of more). The theory is that:


the low probability of switching from non-prion to prion state allows for many more mutations and variations to accumulate -- generating greater genetic diversity than in standard expressed gene variability where most mutations are silent or detrimental
the prions provide a ready form of non-permanent inheritability that can be "trialed" by offspring and others in a colony of organisms -- this can be especially beneficial during say temporary changes in environment
if the prion phenotype is widely successful, selective pressure can easily mutate it into a more permanent fixture in the genome.


Check out the excellent paper published just last week in Nature exploring this this topic [2]. 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.

[1] Crow, et. al. 2011. doi:10.1016/j.semcdb.2011.03.003

[2] Halfmann, et. al. 2012. doi:10.1038/nature10875