Ah, the pleasant delights of content negotiation.
Taken seriously, content-negotiation is powerful stuff, but most W3C specs seem to either ignore it or go out of their way to avoid it. At first I thought that meant there was something wrong with the W3C specs, but now I just think it means there's a powerful integration issues that no one's been willing to sort out - and maybe jettisoning conneg would be a lot easier for most of the parties.
At the same time, though, putting multiple types of content behind a single URI has gotten much much easier lately, partly thanks to XML and XSLT and partly thanks to the strong foundation provided by Apache itself and frameworks like Cocoon.
Time to turn off my brain for a bit and enjoy the new year as it arrives.