Joe Wilcox is right: Apple used a standard move from the PR playbook to push the really bad stuff down the agenda. In announcing that Apple would no longer be attending Macworld Expo and – almost as an side – that Steve Jobs would not be doing January's keynote speech, was trying to push "Death of Macworld Expo" up the news agenda at the expense of inevitable speculation as to the state of Jobs' health.
I know no more about how well Jobs is than anyone else outside his family and inner-circle of friends and colleagues, but I do know this: the kind of illness and surgery that Jobs had means reduced energy levels for anyone, and for someone as actively involved as Jobs is in his business, that means tough choices have to be made about priorities.
Jobs' keynotes are so brilliant as to appear effortless. I've been lucky enough to see Jobs on stage ten or so times, and at no point do they feel staged. As anyone who's good at presentations will tell you, that degree of casualness takes vast amounts of time in preparation and rehearsal to perfect. In fact, it's worth reading former Apple product manager Mike Evangelist's excellent account of preparing for a Jobs keynote for an idea just how much hard work is involved.
My guess is that each keynote speech takes two or three weeks of high attention from Jobs, with days of writing, editing, honing and actual rehearsal. From my own experience of big, important presentations I know that the energy levels required are huge.
If I was an Apple stockholder, I'd look at it this way: do I want a Steve Jobs who needs to take it a little easier than before spending weeks-worth of energy on a presentation, or on honing and developing the products and marketing strategies which have made the company billions?
If Apple needs to see less of Steve, then that decision is a no-brainer.