Well, I was reading a forum thread about some anime decals seen on cars at comiket, and that's interesting, I guess, but I then got to looking for full professional jobs - fully painted bus advertisements.
I found more than I'd seen before and they're kinda cool so I thought I'd preserve the links here.
There are lots on http://zbzbm.hp.infoseek.co.jp/kikaku/moebus/
Chiyo-chan and many other Azumanga Daioh buses
Gunslinger Girl bus
various anime and manga advertised on this bus... 2x2=Shinobuden, Azumanga & Yotsuba to, Onegai Twins etc
DearS
Fushigiboshi no Futago Hime
Lucky Star bus #1
Lucky Star bus #2
Lucky Star bus #3
Sister Princess bus
Futakoi bus
Dragon Ball Z bus
Precure Splash*Star (argh! my eyes!)
the Rozen Maiden Shinku Liner ^_^
and....
the Pokemon Jet
and the Ultraman train!! (ok they're not buses)