Sony had indicated not too long ago that they are interested in adopting other formats on the Reader, which likely implies or at least includes MobiPocket. But it seems Amazon is either not interested in sharing, or not interested in investing resources toward that end.
Wouldn't you like to be a fly on the wall and hear the inside scoop on discussions of that type!? Unfortunately, barring that extreme form of horror film metamorphisis, we'll just have to guess based on obscure hints from the actual players.