http://homepage.mac.com/nilomarabese/
Even though Unity already does this, I thought it was interesting because it does it on the GPU, unlike Unity.
They use an ARB fragment program, but I think it should be possible in a multitap ATI fragment shader as well, no?
Okay, so I haven’t totally figured out how to implement one of those, but that’s for a different thread I think. 
Perhaps this technique would be useful for people who want to generate normal maps in real time in their game or in Unity to get a better preview. What could you do with a modifying normal map generated on the fly… hmm…
-Jon