StarSan is acid based, it destroys cell membranes. This quote came from another brewing forum:
"Yeah star-san and Iodophore are not in the same boat as antibiotics. You can kindof think of the difference like this. Antibiotics usually attack a particular biopathway. For example some stop the bacteria from producing a particular amino acid or someting that the bug needs to live or reproduce. If the bug finds a new way to make that AA they can survive, and since Bacteria collect DNA like Baseball cards this can happen pretty quickly. However for stuff like acid or Alcohol, the mode of killing is more physical, they usually rupture the cell in some way. I believe that Alcohol actually comprimises the cell membrane. Its much MUCH harder if not impossible to evolve a way around this."
Again, if you're worried about it then by all means rotate your sanitizers. I've used StarSan exclusively for the past 3 years (60 batches) and have never had a problem. YMMV.