The evidence for nutrasweet causing damage to the brain is sketchy at best and certainly not of the quality of good medical trials.
As I also said in Michael J. Fox, Aspartame, and Parkinson's disease, the phenylalanine that aspartame is broken down into in the body is also present, in much higher concentrations in quite a lot of naturally occuring substances ...
I wouldn't worry about these scare tactics to try to convince people regarding the dangers of aspartame ... having said that, I'm not a big fan of "diet" anything myself. It's probably safer to avoid Pepsi and Coca Cola altogether ...