You made me search. The correct equation is 1 / (a + bx + cx^2) and the derivation of the coefficients a, b, and c can be deduced from simple geometric models and assuming that the distance, x, is with respect to a non-point light, but something having an actual volume and surface.

For a sphere light, see this beautiful demonstration:

http://imdoingitwrong.wordpress.com/2011/01/31/light-attenuation/

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)