per-pixel shading

Okay, I know this isn’t the best place to ask a question like this, but flipcode msg board is down right now and I need an answer as soon as possible.

I’m looking to do fast-phong shading(using a taylor series expansion to approximate the cos of the angle between vectors), but I was wondering if anyone had a good reference page about it or could help me with some pseudo-code, as the best I can find is non-descriptive, giving an intermediate equation, saying use a taylor series expansion, then use forward differences, and alas, it reduces to two additions per pixel. (what is forward differences?!?!)

Anyone, anyone…? thanks.