I have to add my two cents to this discussion, because one point hasn't been made yet:
I'm absolutely no expert on library sorting systems, but a quick (single-digit minutes) research on Wikipedia seems to suggest to me that the Dewey system is well and good for english- language libraries, but may be too specific for other languages. If any sorting system should be integrated into Calibre, I believe it should be one that supports at least the major languages well- off the top of my head I would say english, spanish, french, german at the very least, maybe russian and portugese as well. That should cover at least a large amount of the userbase, I believe.
That being said, I personally have no interest in such a system, but i wouldn't mind it being implemented either way. It might help in classifying my textbooks, but then again, my tags already do that well enough for me. If it were implemented, I might use it, but then again, I might not
