I must disagree with snopes. Try a little experiment. Get an EXPIRED can of Diet Coke or let a can sit in the sun for a few days. Then, open the can and taste it. It is an experiment you won't want to repeat. The aspartame absolutely deteriorates in the can into methanol. Of course, no one would actually drink it that way unless they were VERY thirsty (in the desert?). I personally would not use anything as food that could break down into a poison. This breakdown occurs gradually. The longer the can has been on the shelf or in the sun, the more breakdown product.
-----
3.3.1.2 Health Issues - Methanol is extremely toxic.Exposure to very small amounts of methanol (less than one cup)have been known to cause blindness and permanent injury including death. Exposure to methanol can occur through inhalation of vapor, or through ingestion or skin contact with the liquid fuel.
http://www.solardome.com/SolarDome22.html (this is not a site about aspartame).
-----
The following is from the official aspartame site to debunk claims that aspartame is dangerous.
5. When aspartame-containing beverages are left at high storage temperatures, the aspartame can degrade and form small amounts of methanol.
http://american.aspartame-info.com/media/rumors/squill.html-----
Has your diet Coke been stored at high temperatures and do you want to ingest methanol?
----
Background: Methanol is a commonly used organic solvent, the ingestion of which has severe potential ramifications. It is found commonly in antifreeze, perfumes, and solvents as well as in the production of formaldehyde. Toxicity occurs as a result of intentional overdose or accidental ingestion. The initial medical sequelae are often the most dramatic.
http://www.emedicine.com/neuro/byname/methanol.htm-----
While most of the literature discusses acute toxicity, in a substance that can be as toxic as methanol, I don't ever want even a tiny little bit of it. Incidentally, I have a confirmed diagnosis of multiple sclerosis and I haven't had a major relapse since quitting aspartame (over three years). I had at least two relapses a year prior to quitting aspartame. A personal anecdote, certainly, but enough for me.