Daring Fireball examines the issue of Quark XPress vs Adobe InDesign today: The Sticky Business of Page Layout. He wonders why InDesign isn't catching on faster. I'll tell you why: inertia. Objects at rest tend to stay at rest until acted upon by another force.
Designers (anybody who puts stuff on a page) will stick with Quark mostly because it is familiar. Sadly, they will probably stay with Quark until they are forced to get InDesign due to company policy or migration to Mac OS X or continued delays in an OS X version of Quark. Let me tell you, though, many people in the industry are taking InDesign training to prepare themselves for the eventual switch. Those who already have are very pleased. Stop the stagnation and get yourself into InDesign today! Please. Don't worry about your service bureau or prepress shop -- if they want your business, they'll support InDesign files or they already do and you don't know it yet.