I know everyone is going to speculate about Steve's health being the cause, but I think it's more likely a result of (in no specific order):
[1] Apple no longer focusing solely on the Mac platform. Let's face it - most of the news these days is about the iPhone or iPod anyway.
[2] Intel platform means there can't really be too many bombshell hardware announcements, as the roadmap is clear and competitors are quick with similarly spec'd kit.
[3] Apple is a fan of special events, and dropping out of MacWorld frees them from having to have something spectacular to unveil every January, and frees us of the disappointment when what is shown is less than we want or predict.
All in all, a sad day, but perhaps not the end of the world.