Wired has a great interview with Bram Cohen, the creator of BitTorrent. I've seen this stat a few times in the last month, and it's pretty staggering:
Analysts at CacheLogic,an Internet-traffic analysis firm in Cambridge, England, report that BitTorrent traffic accounts for more than one-third of all data sent across the Internet.The stuff that's being traded via this protocol is in large part video. These are files that take hours or days to transfer (350M for an hour of HDTV). Sites like tvtorrents.tv make it easy to find just about any television program that has aired in the last year. The BitTorrent client makes moving these sorts of files around possible.
One interesting thing about Bram's BitTorrent client, is that it's just a file transfer agent. Unlike Napster or any of its kin, the software Bram wrote is entirely about file transfer. No searching, no music playback, no library management. As such, it's a lot more like an FTP client than it is like Napster. It doesn't have any predisposition about the types of data you should be transferring. In my book this shines a bright light on the issue of culpability of the programmers creating this kind of software. All they've done is extend the state of the art for file transfer; it's hard to imagine how anyone could criminalize that.