Most poison, even cyanide, in the correct moderation can be harmless. It's when you are intaking sizable quantities of it where it can be lethal.

So, will you die if you have a Diet Coke now and then? Well... yes, we all die. But will it be the soda? Probably not (in fact, I haven't really computed the odds, but I'm sure they're over 1/500,000).

But, if you have soda every day of your life, every meal, don't think it doesn't have an effect on you, even if it's "diet." I used to have a job wherein I maintained soda fountains. You wouldn't believe the filth that coagulates in those spouts. Don't wanna drink soda ever again? Clean one of those things!

On a tangent-- I would be willing to say, having been a waiter in my long line of professions, I'd be willing to bet that nine times out of ten, anyone who happens to be drinking Diet soda who is a sudden fatality usually has other health issue to worry about. You wouldn't believe what some people would order from me (ie. the highest-calorie, fattiest, chemically laced foods ever) and then ask for "a Diet Coke," usually in a large size. Yeesh!