Apple released MacBook Air and MacBook Pro Update 2.0 on Thursday, but that update has only been accessible intermittently. The update improved graphic performance and USB device compatibility, and is still listed on Apple's site, but the Download link results in an error as of this writing.
TMO readers reported receiving an error message (below) earlier on Thursday. We tested at that time and found the link worked, but an hour later that had changed.
Apple's Error Message for Downloading the Update
Since Apple hasn't officially pulled the update, it's unclear if it's a server error or a problem with the update itself. Since the KnowledgeBase article is still posted, the former appears more likely.