Opacity Mapping

Does anybody know any useful links for creating a material with an opacity map that actually affects the object the material is applied to?
I can create an opacity map that affects the texture, but I want to make part of the object invisible at the same time.
Most of the problems I have come across in Blender have been solved with persistance, but I am finding this one to be particularly frustrating and elusive.
Any advice is much appreciated.

You can map that texture to Alpha in the MapTo tab in F5 or you can use Stencils:



Thanks, i thought that stencils might have something to do with it.