Leslie Ellis at Multichannel News has an article (dated: 5/8/2006) called BitTorrent’s Swarms Have a Deadly Bite On Broadband Nets. There was a study that came out stating that "Based on the research, conducted by Terry Shaw, of CableLabs, and Jim Martin, a computer science professor at Clemson University, it only takes about 10 BitTorrent users bartering files on a node (of around 500) to double the delays experienced by everybody else. Especially if everybody else is using “normal priority” services, like e-mail or Web surfing, which is what tech people tend to call “best-effort” traffic."
I've used bittorrent and it works very well. I have and do use VOIP. I know that the guys at bittorrent have been trying to work a business model for their software. I see these 2 as competing goals. For example, at home I use M0n0Wall firewall and in the firewall I have VOIP traffic setup for priority. I think that you will see more of the Broadband providers implement their EULA when it comes to bittorrent and start slowing those connection down.
It will be interesting, I know that in the past some ISP specifically traditional Teleco's and cable networks have been surpressing or traffic shaping VOIP traffic that is not theirs. I think they should probably be looking more at Bittorrent and more P2P network protocols.