Short anwser: no, you cannot.
Long answer: normally, we don't let you change your login name because your user name is part of your profile URL. That means that when the name changes, all links to your profile in people's web pages or bookmarks would no longer work correctly. They might even link to a completely different user when someone else registers with your old name!
Wait a minute. Normally? Yes.. for the time being we will be lenient. Drop us a note and we'll change the name for you!