A new study is showing this to be completely true. In fact, it's showing that "As few as one diet soda daily may increase the risk for leukemia in men and women." Personally, I do my best to avoid Aspartame, the ingredient studied in this case. It tastes funky and gives me a migraine, but after reading that it ups my risk of cancer, I'm happier than ever that I avoid it!
Read the details of the study: