Spider-Man 4 was originally set to come out on May 5th 2011. Marvel Studios and Paramount Pictures have moved Kenneth Branagh's Thor is it's spot. That's two weeks earlier than when it was supposed to come out. Disney then moved in on Thor's old spot of May 20th 2011 and filled it in with Pirates of the Caribbean: On Stranger Tides. This puts it one week before Memorial Day and the release of Todd Phillips' The Hangover 2.
Just to clarify Sony has not yet abandoned their Spider-Man 4 release date, they still hope these issues are resolved and they will still be able to make it. That still won't stop Paramount and Marvel from sticking Thor in the date. They aren't going to wait around for Sony to decided what they are going to do. If Spider-Man 4 keeps the date then Thor will most likely move the release date again. However it can't go back to May 20th because Pirates 4 is sitting their now.
2011 is going to be a great year for movie goers! Also being released that year is Transformers 3, the final Harry Potter, The First Avenger: Captain America, and Green Lantern.