Forgive my ignorance, but it's not like that methyl group just becomes a free radical, right? Does it stick to either the aspartate or the phenylalanine, and if so, wouldn't that potentially change the body's reaction?
/s
In small amounts, your body can handle methanol just fine. Which is fortunate, because it's present in many foods at much higher levels than what you'd get from drinking a can of Diet Coke.