Straight from Mac OS Rumors.com:
According to some of our oldest and most reliable sources within Apple"s software development sector, Mac OS X 10.5 "Leopard" will include a system-level "BitTorrent" filesharing client that can be user-customized to "donate" upstream Internet bandwidth for things like pushing Software Update packages to Leopard users, delivering iTunes Store content, and just about any purpose to which Apple puts its bandwidth.
The code in question was only recently developed, as part of a proposal that would probably not be part of the "Leopard Preview" delivered to third-party developers....but rather would be added to the operating system just prior to the "beta" stage that will follow later in the year.