I just did a shift-reload on both the main Facebook page and the sending messages page and now it looks normal and the Send button works. It might be that it was using the wrong CSS file in the cache or something, or else they reverted the site just as I was shift-reloading. :) Anyway, try that and see if it works.
It's not just you. And it's doing it in multiple browsers, too. It has also broken the "send" button on the Compose a new Message page. You would think they could test the new stuff offline before going live with it.