Thanks everyone for sharing your test results. We've narrowed this down and believe it's isolated to Type 1 fonts used in documents created on Tiger or Leopard, then opened in Snow Leopard (so the bulk of your work, I know). The root cause is in a Mac OS API that we use to read the font's metrics. This issue is not new to QuarkXPress 8.1, it's new to Mac OS 10.6 and also affects QuarkXPress 7. Apple is aware of it and we expect a fix is forthcoming. Our engineers are investigating whether a workaround exists and I'll post an update when I learn more. All Snow Leopard testing of 8.1 was done on pre-release builds of the OS. I don't yet know when this issue was introduced, but I can only assume it cropped up shortly before release.
A few of the issues reported in this thread may also have to do with a runaround problem we just discovered in QuarkXPress 8.1. When runaround is used in conjunction with tabs or lock to baseline, in some cases it can cause reflow in documents opened in 8.1. As a result we're going to release an update to 8.1 shortly. Keep in mind this release won't resolve the Snow Leopard issue described above, but will resolve similar issues for anyone who's experiencing them on Tiger or Leopard.
Product Manager, QuarkXPress