I'd been meaning to check Relax NG out properly for a while, but all the technicalspeak about stuff that's currently way over my head (mostly the hedge automata part, if anyone understands that and can explain it in normal terms I'd be happy to hear about it).
Thankfully, the Relax NG TC (Oasis TCs -- Technical Committees -- are vaguely equivalent to W3 WGs) put up a nice tutorial there.
And it would appear that Relax NG is indeed as cool as it gets. No XML Schema bloat (and bugs), no DTD silly syntax and limitations. It really looks good.
If I have the time, I might even put together an implementation.