I really don't understand the WHO here. We're not talking about saccharin or cyclamate, we're talking about aspartame! Its formula is rather straightforward: a methyl ester of an aspartate-phenylalanine dipeptide. In acid medium (for example, human stomach) it is hydrolyized to methanol and the two individual aminoacids.
And it has some good drawbacks. It's poison to those with phenylketonuria. And, it's hydrolyzed to methanol, which again is incredibly toxic. So it would be toxic, especially in high quantities... but carcinogenic? Then we would stop eating proteins, because phenylalanine and aspartate are carcinogenic? Doesn't really make any sense to me.
Anyone who claims that aspartame is linked to cancer should offer at least a plausuble explanation of its alleged carcinogenicity.