Vocaloid demos I listened to tonight sounded really good, and they're all based on typed sentences according to a post on Federated Media titled: Software Creates Songs, Vocals From Text.

According the the Vocaloid site:

"…(Vocaloid) synthesizes singing exactly by simply typing in words and music notes on your PC. The sung vocal can be output as a Wav file, so it can be imported to other sequencers and played alongside the accompaniment."

The technology is intersting but a little over my head; still I like the idea of what Vocaloid does.