we have the front bike racks on our buses here in Vancouver. i usually put my dahon jack on the front rack, but wil carry on my mu SL. the one time the front racks were full, i folded my jack and brought it on board, and even though the bus was half full, the bus driver mumbled something like "that's a pretty big folding bike, thats what the bike racks are for." i glanced out at the full racks and then back at the driver, but i didn't back talk, and he did let me on anyways.
on board, i parked it in the disability/seniors area where the strollers/wheelchair riders would normally sit. didn't seem to bother anyone on the bus.
interesting was the bikers who were on the bus thought it was pretty cool they would not have to keep watch of their bikes on the racks for fear of someone stealing them.
but again, i would not normally bring my 26 inch jack folded on board. just a bit too clumsy/large for the confined space.