Since I posted this I've done a bit more research on Google. The general consensus seems to be that, while caffiene is a diuretic, it doesn't dehydrate nearly as bad as some had believed. And there is so much water in a typical soft drink (almost all of it), that the effect of the caffiene is small in comparison.
Caffienated drinks ceratinly don't rehydrate you as well as water or Gatorade, but they don't DEhydrate you either. At least, not according to many of the sources I checked. Here is a notable exception:
http://www.freep.com/news/health/qdiet14.htm.