Aside for the beneficial wetting and foaming action, the surfactant's sanitizing job is accomplished by lowering surface tension, which ruptures the cell walls of the microorganisms and kills them. The mechanism may be, by making the cell walls permeable they can soak up water through osmosis, then they explode. The acid also prevents growth of most molds and bacteria.
I've read similar by S. cerevisiae (on hiatus) over on the AHA forum. Essentially, the surfactant (Dodecylbenzenesulfonic acid) is the sanitizing agent of Star San, and the lowered pH of the solution (provided by phosphoric acid) simply prevents multiplying of yeast and bacteria. As he further describes, the surfactant is effective against anionic or negatively-charged cells only (bacteria);
not effective against ionic or positively-charged cells (molds, yeast). He describes it's mode of action as simply being able to permeate the bacteria's cell walls and disrupting (wreaking havoc) cellular function - that is, of course, as long as the star san can contact the bacteria (biofilms come into play here). In addition, I've read elsewhere, that star san does not have a highly effective cleaning mechanism and cannot dissolve biofilms which makes it a poor "cleaner"; whereas, bleach does have a high cleaning mechanism and CAN dissolve biofilms making it an excellent cleaner (and sanitizer too). In comparison, iodophor is effective against both anionic and ionic organisms making it a more robust sanitizer but still a poor cleaner. Peracetic acid (of which a lightweight version can be easily homemade) is both an excellent sanitizer and cleaner, as well.
So, as I currently understand it, star san is effective against most bacteria as long as the bacteria is not protected by a biofilm. All other possible beer spoilage organisms are not affected by star san - simply restrained until pH increases.
As for me, while I find the information extremely valuable, I still use star san as my primary sanitizer for most purposes. When it comes to cleaning sour-beer contaminated equipment I use boiling bath of oxyclean solution, followed by star san. Probably better would be to quarterly (or semi-annually) put my equipment through a bleach and/or iodophor bath to provide more robust protection against long-term, slow-growth contamination.
None of this is verbatim; it's all para-phrased and any mistakes on the regurgitation of information are my fault.
Cheers!
Edit: To test the theory of star san being ineffective against yeast (yes, yeast); simply harvest a half-jar of some yeast from your next batch and fill the other half with star san. Shake it up and give it some time in the fridge. On your next brewday, pour a quart/gallon of your freshly chilled wort into a test fermenter and then add your acid-washed yeast (i.e. star san yeast) and watch it ferment away.