OK I broke down and looked around on the internet. The AP Stylebook (long may it reign o'er the scurvy mass media) says no to the extra s after the apostrophe on singular possessive proper nouns ending in s. Goober, however, is an Elements of Style man, and they say add the s no matter what or there's gonna be trouble. I was wrong according to most of the places I looked about not adding s on singular possessive non-proper nouns ending in s, so "bus's" is indeed legit, as is "hippopotamus's". The only style manual I hit up that supported me on that point was one by Mary Ansell, which said that it was acceptable in American English to use just the apostrophe, but I had never heard of her book before. Getting back to the main point, if you refuse to make exceptions for proper nouns you will end up breaking the only really important rule of the English language: Try not to sound stupid. I submit for your reading out loud pleasure:
Joan Rivers's sisters. Hercules's seas. Sisyphus's Syphilis. Matt Damon.
So now I am a proponent of just writing it how you say it and respecting the author's vision by saying it how it's written. No matter how much I hate looking at the words Bridget Jones's Diary. This probably should be split off and deported to a board that is more supportive of tangential thread derailment.