In cases where you need to interpret the resulting model, I've been advised not to bin (for example: http://biostat.mc.vanderbilt.edu/wiki/Main/CatContinuous). Other alternatives are splines or generalized additive models.
However, people's rationales for why they should bin is often that it makes the model better / more interpretable, without actually testing the more restricted binned model against the more general one. There's certainly something to be said for knowing your audience when choosing a model, though :).