I'm a bit at odds with most opinions - I think it's quite conceivable Lehmann may not have known, but bollocks to the bowlers not knowing
The only scenario I can think of where the bowlers were not involved nor the coach is if the batsman took it upon themselves to try and get the ball changed because it was in such bad condition. I still think this is highly unlikely. The best that can be said about Lehman is he knew but turned a blind eye.
If he honestly didn't know what kind of a set-up is he running there? He should resign for being clueless in that case.