It depends on the type-age of the bus. The local busses in Orlando I would guess are all very new diesels, they don't make any smoke, and not much smell (now, the bus service is something else, it is pretty lousy out in the suburbs). OTOH, school districts often are strapped for cash and can't afford to upgrade their very old busses (usually they don't have EGR, catalysts, or even turbos- all things that can cut down on emissions).
The diesel fuel itself has also gotten better. Sulfur in diesel fuel used to be very high, in the early 90's they cut it to 200 parts per million or so (with initially disasterous results for fuel pumps, from what I've read).
Gasoline has sulfur too, and it too will be lowered. It's just gasoline engines, at least the ones most commonly sold in the US, don't produce much nitrogen oxides on their own because they burn relatively inefficiently without a surplus of oxygen, so they haven't needed a catalytic converter to reduce nitrogen oxides. OTOH, the Honda Insight has a lean-burn V-TEC, and generates alot more nitrogen oxides than many nonhybrid vehicles with more horsepower. The future of gasoline engines, though, is likely going to be leaner burning engines, so they will get catalysts too.
