There are a few programs that (claim) to do this. Some work not too badly IF:
1. there is a single melody line (no harmonies)
2. the notes don't overlap - not a problem with a solo singer.
I don't use this feature but have been using Powertracks for about four years now. Several people claim success - I tried it once on a midi-generated sax line and it was not very good. I believe the software can be tried out before buying. In any event, it is very reasonable.
PT is the little brother of Band-In-A Box. http://www.pgmusic.com/index.html