I've updated my entries on hydration. The overview article has some tweaks and the detailed article has a new section giving details on sodium losses while racing. There is also a table of sodium losses for sample times, sweat rates and acclimation levels.
Overview - http://fellrnr.com/wiki/Practical_Hydration
Details - http://fellrnr.com/wiki/Hydration_101
Sodium Loss table - http://fellrnr.com/wiki/Sodium_Loss