Dehydration can affect a person by decreasing their sweat rate; increasing their core temperature, increasing heart rate, and decreasing blood flow to the skin (among other things). 

The symptoms have little effect on the armband's ability to estimate caloric burn, especially in the big picture. In other words most likely someone using the bodybugg would not be in a dehydrated state for any extended period. Additionally, if the symptoms outlined above actually reached levels that may affect the bodybugg^''s reading, the individual would be near collapse.

The science from the bodybugg lab:

The bodybugg team tested the ability of the armband to measure dehydration as part of the development process.  The changes caused by dehydration do not seem to greatly affect the predictions made by the armband's algorithms.  Largely, this is due to the fact that the armband looks largely at relative changes in sweat levels.  Dehydration happens at a slower speed than changes in sweat due to exercise and, furthermore, factors like humidity have an even bigger impact on perspiration rates.  These factors tell the bodybugg team to utilize measures of relative change rather than absolute values.