There's a gaming magazine on Teletext in England called Gamecentral (formerly the late, great Digitiser). Every weekend they have a topic to discuss on the letters page, like 'what's your favourite Transformer?' or 'How do you feel about the PS3?' Every week, regardless of relevance, someone writes in asking for Shenmue III. Makes me laugh.
I don't know if anyone out there's got any facts, but as far as I know the games are too damned expensive to make and wouldn't be expected to sell well enough to recoup it. Given that Yu Suzuki originally wanted to make eighty-three Shenmue games to tell the story (I may be exaggerating ever so slightly) it's difficult to imagine that he'd satisfy himself with just one more (Shenmue Online aside). Personally, if it made a difference, I'd buy two copies. I need closure.
Plenty of online petitions floating around, though, for all the good they do.