A big reason I think they're de-emphasizing MacWorld is the crazy swings it brings in terms of stock prices and expectations. Apple isn't going to introduce an revolutionary iPhone year after year, and year after year AAPL goes through massive trade activities with speculators. We started holding up a MacBook staring at the share price every year and watch it go freakin' crazy.
Secondly, old media is dead. Long live new media. New media that we have in place -- online news, pro and amateur blogs, twitter, rss, facebook, chat -- has easily outstripped the effectiveness of things like trade journals and trade shows.
Thirdly, MacWorld has always been an odd compromise internally. Preparation for it begins early but with that pesky Exodus (12/22 - 1/2) right in the middle, it puts a rush on the rest of the 5 days to polish all of the marcomm materials and presentations. Then you have the devs all stuck in SF for an entire week.
If MacWorld was held at a later date then the costs might be justifiable -- it's why Apple left MacWorld Boston. The catch though is that MacWorld can't be held at a later date lest it be trumped by CES. Either way, not showing up Macworld is going to decrease the enormous strains an arbitrary January date puts on the company. The Town Hall meetings are perfectly sufficient, a lot closer, a lot cheaper, and a lot more controllable.
Also, Steve Jobs not doing a keynote isn't a big deal. Schiller will not be his successor (I thought at one point it was going to be Forstall but that idea went out the door), but if there's no big products to push, Steve might as well just hand it off to his lieutenants. If and when Schiller announces an Apple product, then be worried.