Not sure what it is, but I have the same reaction when it comes to diet sodas -- with the exception of Dr. Pepper. Not sure if it is the additional spices in it or not, but a number of times I didn't realize I was drinking the diet version.
Dr. Pepper seems to have less aspartame. It's the only diet soda I can drink without gagging (though I'd still normally avoid it). I can still taste it, but it doesn't bother me as much.
Pretty much a moot point since I don't drink much pop to begin with...
