This is a shader that mimics boolean mesh operations in the pixel shader. It is very fast ( ~ 200 subtractive meshes @ 60+ fps on my system).
The result of the operations is not completely correct, but looks believable enough. Best results are obtained when the operands are convex. Because it runs on the pixel-shader, the system is not influenced by mesh topology, polygon density, etc.
More information and some code: johanholwerda.nl/blog/?p=116