I think Apple is understandably upset at IDG for moving the show to Boston:
1. Media/publishing/creative presence is a lot stronger in NY than in Boston.
2. MacWorld Boston doesn't feel as 'glamorous' as MacWorld NY, especially to us non-US residents. I don't mean any direspect to the Bostonians out there but just the fact that there is New York in the name gives it a special ring, as if to say it is for the whole world (anyway that's how it feels to me).
3. The fact that (i guess) people are more enclined to travel to NewYork for other reasons, so they can kill 2 birds with one stone.
That being said, i don't believe that Apple will stay away from that tradeshow: i think they are trying to renegotiate some deal with IDG and using this as a sort of threat to get what they want... In a way it backfired with IDG threatening to keep Apple out of MWSF, but then who is going to believe that IDG would do it anyway (that would be pure suicide).
All in all, i don't think that the tradeshow makes a massive difference to the Mac community. Most of us are fanatics that follow Apple's every move on the Web and through a multitude of Websites. We don't wait for tradeshows to get news.
I think Apple is trying to give its presence at those tradeshows (including SJ's keynotes) a higher profile (like when Bill gates presents his new OS or TabletPc or whatever),especially for the non-Mac users (switchers, anyone ?), and they probably believe that it is in NY and SF that this can be achieved.
My 2 cents.
Nico