There's a good documentary on Hatsune Miku here - it's in Japanese but with English subtitles (click the 'CC' logo in the bottom right of the YouTube player to turn them on): http://www.youtube.com/watch?v=xBZOlipfjkQ

I find the Hatsune Miku phenomenon somewhat amusing at the moment, given the trend of overt Autotuning in pop music - while virtual singers' voices are getting more realistic, real singers' voices are being manipulated to sound impossibly accurate and digital.

It's also an interesting study in image and marketing: Crypton really pushed the concept of associating a character with the voice, as if Vocaloids were virtual pop stars whose services you could buy, rather than just another virtual instrument.