well, I should clarify a few things I said:
First, when I said "it''s happened before" I was referring to Street Fighter. Can't remember the character name.
Secondly, when I said "clone," I meant an official character with a different name that looks similar to this fan-made one. Not enough to count as "stealing" the character, but enough to excite fans of the fan-made character.
Lastly, you raise a good point with the intent of the program. Yes, they do indeed encourage user-made content, unlike -- heh -- Street Fighter.
But if it's legal, fair, and there's profit in it, the company that makes Vocaloid might still consider incorporating a particularly popular character. I'm just thinking of how they took the popular real life star Gackt and made his voice into "Gakupo." I actually think the idea of taking cues from fans is pretty cool, and I usually love it when it happens.