@colin_:
I've personally been formatting the package documentation with LaTeX, which can be exported to a wide variety of formats (dvi, pdf, html, etc. etc.). Not sure what everyone else thinks of it, but I thought I'd throw that option out there.
If we're doing something like that, I'd rather stick with DocBook. Problem with either of those is the potential contributor base is much smaller than if you use something like a wiki. Just because people don't want to mess with it.