A colleague, Christophe Lauret, sits a few desks down from me. He has been working on the PageSeeder web-based collaborative environment, and he called me over last week to show me a really interesting new use of Schematron.
It looks slick, acts fast, and provides pretty powerful validation. The kinds of HTML documents being edited are relatively simple, so the implementation did not need optimization.
I'll post some extra info when it comes to hand.