Until QuarkXPress goes native I don't see the majority of Mac design users making the switch to OS X.
Sure, they can switch to InDesign. InDesign does everything better than Quark. And you can find a service bureau that will take your InDesign files instead of Quark, even if that means switching a relationship you've have for several years.
But, if you've got a decades worth of work in Quark, it's going to take a pretty big push to get you to switch.
I work in the prepress department of a big midwestern printer and we don't get a lot of InDesign files. The ones we have received often give us problems (font and color issues mostly). InDesign 2 is a vast improvement over version 1, but there are still some kinks to work out. And the bigggest problem is that our imagesetter company, Creo/Scitex, hasn't really updated its ripping software to deal with InDesign.
That's a major consideration. When a company spends the megabucks it takes to bring Creo into a shop and Creo has built their equipment around support for Quark, it's had to be motivated to change even if a lot of customers are demanding that change. Creo blames Adobe for not giving them the technical information soon enough to be able to incorporate changes into their equipment. In any case it leaves service providers in the middle with customers interested in changing to a new program and not being able to because the service providers have equipment that isn't fully compliant with the new software. A case in point: the transparency features in Illustrator which will choke a rip.
Until all players are on the same field, it's going to take time to change.
Dennis E. Henley