As usual after every keynote, Apple has posted a first video of the event, available for streaming on Apple's website. You can reach the video here, or by pasting this link in your browser's address bar: apple.com/apple-events/october-2011. Apple notes "streaming video requires Safari 4 or 5 on Mac OS X Snow Leopard or Lion, Safari on iOS 3 or later, or QuickTime 7 on Windows.".
A downloadable version of the keynote should be made available on iTunes in a few hours.
Update: The October 4th keynote is now available for download on iTunes through Apple Keynotes' feed.