Quark PR today released this statement about Leopard compatibility.

“Our early tests show that QuarkXPress 7.3 will run on Leopard, except for
the Flash export in Quark Interactive Designer. We chose to focus on
optimizing the latest version of QuarkXPress because it offers our
customers the greatest value, features and benefits. We plan on releasing a
QuarkXPress 7 update with fully tested Leopard support within the next few
weeks. This will be a free update for existing QuarkXPress 7 users and will
be available to download from the Quark Web site. QuarkXPress 6.5 is not
compatible with Leopard.”

www.quark.com