Here is one of the links I ran across that list concerns with Splenda:
http://www.mercola.com/2000/dec/3/sucralose_dangers.htm
It's hard to tell if these are legitimate concerns, or just the far left, all-natural-all-the-time crowd going off the deep end again. Common sense though, would tell you that if you take a natural product (like sugar) and turn it into something undigestible by humans, there is the possiblity it may cause some problems.
A statement on page 5 was also notable:
Is There A Benefit for consumers?
According to Consumers' Research Magazine, sucralose provides some benefits for the corporations making and using it, but not for consumers.
They state that diabetics, weight watchers and the general public might make better food choices by selecting basic, rather than highly processed foods; for example, apples, rather than turnovers; or plain, rather than sweetened, dairy foods.