It’s done all sorts of half-way steps, gradually going from showing blank text and no cache with an “unknown” media type to caches to plaintext versions with bare tags.
It appears that Google finally understands my application/xml pages. I don’t know if it goes all the way to supporting application/xhtml+xml, but I’ll take what I can get.
This means that if you use the recommended method of serving XHTML to all half-modern browsers without any goofy browser sniffing or text/html, you won’t be left in the web dust. Yay!