Dave,
I think you have the secret. This is a normal thead (not a left hand one like the screw in the chuck). One thing is to make sure the torque is up as high as possible and that the drill is set in the correct way(Fwd/Rvs) so that tension is against your tap else the whole motor body just spins.
FYI on the Bosch web site they carry diagrams of every tool for the last 25 years os if in any doubt you should be able to go on there and look at the exploded view and (probably) download a handbook.
Having said all that I could not get the chuck off my old drill either but fortunately I could get to what I needed with the chuch still attached.
John
June 2008