to demonstrate it even more - i played a little bit with maya. and here is what i done: 4 spheres, 2 lights, and 2 shader. the yelloe spheres have a lambert and no bump, the white spheres are lambert with bump. i choose a pointlight and a ambient light - both are at the exact same postion (about in the middle of the picture). the left spheres are only lit by the point light and the right spheres are lit by the ambient light. to make the ambient light look about the same as point light i changed the ambient light shade to 1. but you can clearly see (and if you dont believe me try it out for yourself) - that w/o bump ambient light and point light act the same, but with bump the ambient light ignores that.

hope that makes the last post more clear