Lawrence points to a News.com story that sez Sobig is aptly named: the fastest spreading virus ever. Guesses as to what made it spread so quickly: a combination of good social engineering (randomly selected forged return addresses) and good spam-filter-busting capabilities (the rotating subject lines, the changing return addresses, the changing attachment name). No surprise: the BBC says that Sobig seems to have been written by a spammer who needed a way to get his messages past spam filters.
Frustrating point about this worm: it really has nothing to do with Outlook. It doesn’t exploit any Outlook vulnerabilities—except maybe the fact that it’s easy to click and execute an attachment in Outlook, and to read Outlook address books. The worm carries its own mail sending engine around with it. And because the worm is so self reliant, it isn’t easy to avoid it—there’s no “magic bullet” patch that will keep it from spreading. Except behavioral changes on the part of users, and maybe switching OSes.