There is no way to do it with iTunes, but there is a tool in the works that will allow you to do it. You just need to be patient and wait for the new release of Redsn0w, which will allow a phone currently at 5.1.0 or 5.1.1 to be restored to the same firmware (provided you have saved your SHSH Blobs). But you must wait; if you upgrade to 6.0 you will never be able to return to 5.x.