The current issue of Computer Music has a blurb about a recent development by Yamaha called the Vocaliod. It is a virtual vocalist that 'sings back' the words and notes you type into it. It is sample based and has a piano roll that allows you to change the pitch of a note. The author heard a demo of the software and called it 'impressive'.

This thing might be no more useful than the singing head', but who knows? Ya gotta admit it's interesting stuff. I did a Google search and found a site with demos:
http://www.soundonsound.com/soundbank/YamahaVocaloid.php