Rusty, Horseman: 'Buses' is used here too. Grammatically, it's the correct way. I don't know, but I think that 'busses' is recognized as well since it must be common to mistake it as correct. Most English singular nouns that end in 's' end with 2 of them, and so their plural keep the 2 's's. and it becomes a tendency, I guess, just by the way it sounds. The only singular nouns that end with a single 's' that I can think of right now are 'bus' and 'gas' (buses, gases)
..but then again it could just be one of those things, like 'tranquility' and 'tranquillity'; and 'special' and 'especial' being correct..