I suspect Apple has been looking for the exit for some time, but I don't think they'd have handled it this way if something else wasn't involved. At the very least Steve would have done the keynote (or shared it with Phil) and announced after the show that this would be their last appearance.
Why? So we could all huddle together and feel all warm and nostalgic and have a moment of closure? What has Steve Jobs ever done to give you the idea he would step away from something like MacWorld with any nostalgia or ceremony? Was it when he slashed through Apple's product line-up when he returned to Apple? Was it the clearing out of the old Mac archival stuff from the Apple campus? Was it the jettisoning of all the beloved Mac-isms from the OS 9 era? Was it his complete lack of acknowledgement of any of Apple's supposedly significant anniversaries? The man clearly isn't as nostalgic about these things as some of his customers are.
Besides, I believe Steve has cut ties with other Mac-related events without any ceremony either so why would you expect him to behave any differently this time?
And frankly, I don't care either way and I don't understand why any of you would need some kind of closure for something so trivial.