I've been looking around on the goold old internet, and it appears that overheating vegetable oils may not, in fact, produce trans fats. There are more pages on the "it does" side, but those that have references seem to be kinda ... flaky. The ones on the "it doesn't" side seem to be more reputable.

On the other hand, just because I don't think that McD's doesn't use hydrogenated oils in their deep fat fryers doesn't mean that they don't. After all, the biggest (only?) plus to hydrogenated oils is that they have a very long shelf life, and the fat in a vat would be one of the things a typical McD's would keep around the longest. But somehow I doubt that the oil solidifies overnight. That would just be too disgusting. But I could be wrong.