Apple is offering a live HTTP video stream of the Keynote today, but apparently only on Apple devices.
Apple has announced to stream the September 1 media event live today using HTTP Live Streaming. HTTP Live Streaming is an HTTP-based media streaming communications protocol implemented by Apple Inc. as part of their QuickTime X and iPhone software systems according to Wikipedia.
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Apple said that you need a Safari Browser on OS X 10.6, iPad, iPhone or iPod touch to view the live stream. MacRumors points to a sample video that is streamed using HTTP Live Streaming on the Apple site. I tried it on Safari running on Windows XP and it did not work. It only works for me on Safari on my iMac.
MacRumors is suggesting that to try VLC on Windows, but it might not work on a live stream.
In a nutshell Apple will not use any kind of user agent detection to block non-Macs from watching the keynote. Apple just uses new technology that is still only an Internet-Draft to be submitted to the IETF to become an Internet standard.
The answer to the question on how to watch the September 1 Apple Media Event on Windows is: It will not work. Unless you are following iHackintosh's lead and install OS X Snow Leopard on your PC via VMWare.