So if you might have missed the event somehow you can now watch/download the event in High Definition from iTunes.
The complete download is around 1.34GB for “normal” resolution. The high-definition version will consume a whopping 3.03 GB of storage.
So just go to iTunes and start downloading the Keynote video. [link]
Or you can also stream the keynote video at: http://www.apple.com/apple-events/september-2012/