Ok so this isn't strictly tech, but I don't think its getting enough press coverage:
Aspartame, the sweetner used in Diet Coke, Pepsi Max, most diet drinks, chewing gum, some yoghurts and crisps (the list is huge) is carcinogenic.
The Guardian recently produced this excellent article on the issue.
One doctor even suggested a full Government conspiracy.
More info on aspartame can be found here, and numerous articles can be found on Google
In the interest of being unbiased here is a report suggesting
Aspartame has no effect, funded by a number of big name chemical companies.