Blindsided is probably an understatement

CNet News has a column by Steven Musil about the demise of Macworld as an Apple-supported event:

After some 25 years, Apple has decided that it can do without the Macworld Expo.
Apple announced that CEO Steve Jobs had given his last keynote address at Macworld in San Francisco and that January's Macworld would mark its last year participating at the show. Apple said Phil Schiller, the company's senior vice president of worldwide marketing, will deliver the keynote, usually handled by Jobs.
An Apple representative declined to comment on Jobs' health, a prominent topic of discussion this year. Jobs' keynote addresses at Macworld have become almost legendary events, launch pads for some of the company's most important products and strategies. His absence from what many in the Apple community consider their Super Bowl has once again revived rumors that Jobs is ill.
The decision reveals an Apple that has decided it no longer needs to make an appearance at the event that has come to define the company in recent years. In doing so, it's also preparing for a future when its iconic founder no longer dominates the stage the way he currently does while confirming a shift in its strategic thinking when it comes to reaching customers.
But industry events like Macworld Expo have been losing their luster inside Apple for some time. This is not a company that spends much time hanging out with its peers in the personal computing and mobile phone industries. And quite simply, the nature of technology marketing has changed a great deal as tech has evolved from something reserved for professionals to something that almost everyone uses on a day to day basis.
The announcement "completely blindsided" IDG, according to a source familiar with the negotiations, coming just weeks before the event was scheduled to take place. IDG World Expo, a division of tech publishing giant IDG, had no reason not to expect Jobs would make his customary appearance at Macworld. Then came the word from Cupertino.
What is not clear is when Apple decided Jobs would skip the keynote. The source indicated that Apple had strung IDG along for weeks, implying that it was business-as-usual concerning Jobs' pending appearance up until the moment that it wasn't. One big question is how this will go down with the Mac faithful who have flocked to this annual event through good times and bad. To be sure, there will be official Apple events in the future, like the Worldwide Developers Conference. But most fans, the civilians, likely won't be able to gain access to such events, which are usually reserved for press, analysts, VIPs, and developers.
So, as one colleague put it, Macworld has long been the public carnival for Mac fans, and Apple's decision to get out after the 2009 version doesn't bode well for Macworld's future or for the future of a single, mass event for the hardcore Mac community.

Rico says Steve Jobs might be impulsive, but Apple never made a corporate decision like this lightly; if they're out, it's for a good reason...