The diet sodas aren't bad on the waist line, but sometimes too much artificial sweetner such as splenda and sweet n low can make it harder to get rid of those last few lbs of water/fat. But we are talking about an extreme contest diet when you are trying to pull it all together that last week. I doubt for the rest of the year or for someone looking to get into good shape that it would make much of a difference.
I forget, is there much sodium in those diet colas? I haven't had one in a while.