There’s an interesting article about mandoc and mdocml up on undeadly.org, talking about its history and usage in OpenBSD. It’s present in DragonFly, though it hasn’t been set to replace anything (i.e. groff), yet, that I know of. I do like the mdocml HTML output, and I’d like to see it here.
A running description of activity related to DragonFly BSD.
Seems to have -Tps now, too, as of version 1.10.1