Matt Haughey has worked up a little analysis/thought experiment of just how much money could be made by buying stocks before every Macworld Keynote of the past ten years-- he calls it the Keynote Index Fund, and has the stock prices before every keynote, directly after every 'note, and a day after every 'note.
And the fact is that buying the stock a day before and holding it for 48 hours (until the day after) would have made you money over the past ten years-- he calculates 1.2% growth over 24-hour period, and 2.2% growth over a 48 hour period. Of course, that doesn't hold a candle to what you would have earned if you just kept Apple stock the whole time (holding on to $10,000 of Apple stock since 1997 would have you holding shares worth $525,187 today).
But the fact is that Macworld keynotes can make wily stock traders money. The worst performing keynote so far was in 2005, when only the Mac mini and the shuffle were announced, and the best was last year, when the iPhone was first introduced. So just standard common sense just tells you that if the iUltraportable does appear, you could probably make money with a little day trading*, but if it doesn't show up (and there are no other major announcements), you could take the worst bath so far on the Keynote Index Fund.
*This advice is given by a nonprofessional and should not be listened to under any circumstances or by anyone-- past performance of a stock means nothing to future performance. Plus, I'm still hungover from New Year's Eve, and in no condition to give stock tips anyway.