The applications for recorded-voice forgeries are obvious, but I think the larger security risk will be real-time forgery. Imagine the social engineering implications of an attacker on the telephone being able to impersonate someone the victim knows.
I don’t think we’re ready for this. We use people’s voices to authenticate them all the time, in all sorts of different ways.
EDITED TO ADD (5/11): This is from 2003 on the topic.