Hyponatremia was discussed at the WFR recert I recently attended. According to the instructor there have been more issues during hot weather marathons from hyponatremia than from heat stroke.

The issue is consuming a lot of water AND not replacing electrolytes. You basically sweat off all the sodium in your system. Because it is hot you don't feel so good, so you drink more water, thinking that will make you feel better. That causes you to sweat off even more electolytes, so you feel worse, so you drink more water .... repeat until you go into seizures. People do die from it. One of the issues is that early symptoms can look a lot like heat exhaustion, and late symptoms can look like heat stroke. However, just giving more water makes the problem worse.

Through the magic of Google I found a back issue of Boatman's Quaterly with a good discussion of Exertional Hyponatremia.

As with most things, the best treatment is prevention. When you are hot and sweating a lot, drink lots of water, but also eat salty snacks to maintain your electrolyte level!