Unless you have kidney or liver issues (which you would hopefully know ahead of time) and you don't drink a ton of it, no it's fine.
But here is a quick bit of info that I found:
"Diet sodas are manufactured with artificial sweeteners - one of the most popular being aspartame. The validity of aspartame as a health risk has been debated for many years - some claim the sweetener may contribute to increasing toxicity levels and risk of various illnesses, while other dismiss such charges."
"What we do know for certain is that aspartame contains phenylalanine, an essential amino acid that does occur naturally in certain dairy foods and green vegetables. On the other hand, one byproduct of aspertame is formaldehyde!"
Sooooooo, there is a lot of conspricy theory going on here, but basically, take everything in moderation (including moderation itself) and you'll be fine :thumbsup:
Here is another good link
Aspartame controversy - Wikipedia, the free encyclopedia
HL