Thor and Spidey are now in a bit of a stand-off. Sony has not officially given up that May 6 date, and could feasibly rush Spider-Man 4 into production in order to make it. If that's the case, Thor would have to find another release date, and probably wouldn't be able to reclaim May 20 as another blockbuster will undoubtedly move to take its place.
But realistically, Sony probably can't make May 6. Thor starts filming in another week or so. It's cast, crew, and script are set. As we know, Spider-Man 4 doesn't even have a villain let alone anyone to play him (or her). Even if Sony and Sam Raimi split up over creative differences, I doubt they can coordinate talent schedules quickly enough. But hey, May 2011 is over a year away and when a studio wants to get something done, they can. But for now, May 6 belongs to Thor.