At a recent meeting of the American College of Sports Medicine, a study on the impact of hydration on cognitive function was discussed.
While most of us would expect cognitive function to deteriorate with dehydration, the study found that reaction time was faster after the dehydrating exercise. Then, after the subjects began rehydrating, reaction time returned to normal. At no point was reaction time worse that in the pre-exercise test.
However, other studies contradict this finding so the jury is still out.
You can read the full report, from Runners World at Dehydration and Cognitive Function.