Apple seem to be moving away from these shows.
There's been a trend in recent years for them to announce new products on their own turf, at their own "events". As Semihere points out, Apple not appearing at MacWorld isn't news, and they haven't always announced much when they were there.
MacWorld—and similar events—are an awkward choice for a major product launch: you really don't want to spring a brand new device on your third-party development and gadget community right in the middle of a show. The result is inevitable: their product hogs all the limelight, while everyone else at the expo gets shafted in the publicity stakes. It also means each company's stall is swamped by idiots asking what they think of the new iHardware. Will their software work on the new hardware? When will it be released?
I wouldn't want to be running the Mariner (Write and Calc) stand, or Nisus' when Apple have just announced an update to iWorks. They might as well just pack up and go home for all the coverage their own—much more mature—apps will get. It's bad for business.