@HarryT: I agree that MOBI as a platform for delivering content isn't going away anytime soon, but that's not the point. calibre already converts *from* MOBI, just not *to* MOBI. My point is that in the very near future most devices that read MOBI will also read epub, which means that converting to MOBI will be pointless, since you'd get a nicer ebook converting to EPUB.
@wallcraft: You could always vote for me to write an epub viewer
Certainly creating a HTML wrapper around an exploded epub should be easy.