I guess most of us have been sold on the idea that we need balanced electrolytes (K, Mg, Ca, etc.) rather than just NaCl, to ward off cramps and such. but on the other hand, if you are needing a ton of fluids, you had better be taking salt wherever you can get it.

I've never heard of the problem occurring for cyclists, but there are cases of fatalities when marathoners drink too much H2O

Marathon Dilemma: How Much Water is Too Much?

(edited text follows here, after a closer reading of the article): It seems that electrolyte intake is important, but that other factors also can cause significant problems when water intake is high. A complicated business. You'd think that people doing double centuries and the like might have to be careful about this, but again, I've never heard of it happening to a cyclist - just marathoners.