Originally Posted by DannoXYZ

That's about sums it up. You can absorb water about 2-4x faster than needed to replace what's sweated away. There's also a tonne of electrolytes that go out with that water. Coming back after a ride, if I don't change and shower right away, my back & jersey is coated with a white layer of salt after the sweat evaporates.

Here's some good articles on hyponatremia that Machka posted in some previous threads:
UltraCycling - electrolytes
Ultra Cycling - low blood sodium
Ultra Cycling - drinking too much
Ultra Cycling - water and salt intake during exercise
Ultra Cycling - fueling for endurance

It's interesting to note that on the RAAM, they don't drink as much as they sweat away; they end up a little dehydrated at the end of each day. This is to maintain a higher concentration of glucose/complex-carb/protein mix in their stomach and get more calories/hr through.