Back when I was using Panther, I occasionally had my Mac lock up when using BitTorrent, which I believe had to do with the fact that BitTorrent opened many connections simultaneously and quickly. Now, lfroen is correct that this is a kernel bug that is Apple's fault. However, to work around it, make sure to use reasonable limits for "Max. Connections" and "Max. new connections / 5 sec.". I'm using 500 and 20, respectively, without problems.