I posted this on the AppleInsider forums, but since many MacNN people don't read AI, I'm posting it here. Apologies for the spam.
I've written a simple little utility that does two things.
1) It displays a countdown timer in your dock, that says how much time remains until the MacWorld keynote.
2) It checks Apple's website for their next teaser message, and informs you when it has changed. (beeps once, and throws up a dialog)
You can download it at
http://homepage.mac.com/mwsf2002/FileSharing1.html.
It's a Cocoa app, so it only runs on Mac OS X. I've also included the source code, so feel free to modify for your own use (but please don't redistribute it).