Most of the good-looking implementations are a bit more complex than you’d expect. For example, you can have a look at how it’s done in Unreal Tournament: link text
Just a note - if you want the weapon to lead instead of lag, this implementation works when you set the MaxYawLag or MaxPitchLag to negative values as well.
P.S. I forgot that I’ve posted the full source code including my small additions a while back on the forums: Weapon Rotation Leading. This is probably easier to use as a reference.