Apple has filed suit against three specific developers (members of the ADC) for putting copies of its new OS, code-named 'Tiger', onto BitTorrent. The story isn't clear on how these three individuals were tracked down. One possibility is that since BitTorrent only hosts pointers to downloadable streams, these people had the actual streams. Another is that pre-release copies of Tiger had some kind of embedded watermark that identified who the original recipient of that copy was. This is an old trick that has been implemented with many technologies, from physical watermarks through digital steganography. In one famous case, the Thatcher government in the UK found out who had been leaking physical memos by causing different copies of memos to have different patterns of whitespace within block-justified paragraphs of text. When the memos were recovered (by subpoena from the BBC, as I recall) it was easy to determine whose copy of the memo had been leaked.