It was terribly convenient, too, that those busses were going by just as the Joker pulled his out.
"It's all part of the plan."
But, yes, it was odd that none of the bus drivers apparently took any notice of a bus pulling out of a building and slipping quietly into formation. Or perhaps they did, reported it, but the Joker slipped away in time to avoid capture?
One part of the film I have never understood is why the kidnapped Dent knocked himself over in his chair, which lead to him knocking over some of the gas, which in turn ultimately lead to half of his face being burned over. What was he trying to accomplish?
And why did Joker think that only Dent or Rachael could be saved? Did he calculate the exact moment in time Batman would learn of the two locations, the exact speed of the Batpod, future traffic conditions in Gotham, and adjust his timers accordingly? Not bad for a rabid dog chasing cars.