Aspartame: True or False?

I was talking to a girlfriend this afternoon and we got on the subject of a book someone gave her on nutrition [I can't remember the name of the book except that it has two words in it and the last word is b**ch] - I guess it's an entertaining, irreverent book that, once read could send you on your way to becoming vegan! It sounds really interesting. Anyway, in one part of the book the topic of diet sodas comes up. The author said that when someone drinks a diet soda, the aspartame turns to formaldehyde while in your body. Has anyone else ever heard that? I've heard before that diet sodas were not good for you - especially for those actually trying to lose weight, but this was a new one for me. I personally have never enjoyed diet drinks - I go for the pure stuff - you know, straight sugar - that's much better for you!