Mac OS Rumours claims Mac OS X 10.5 "Leopard" will feature a system-level BitTorrent filesharing client that can be user-customized to 'donate' upstream bandwidth for things like pushing software update packages to other 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.
Although implementing the code to secure the data traffic and break it up into pieces so small that individual users who enabled Sharing-Reward accounts would be unable to make any use of the data themselves is actually the easy part, getting Apple Legal and the executive suites to sign off on the concept will be a much larger challenge.
Now you're probably thinking why people would decide to donate their bandwidth to Apple? Well, the company thought the same thing and decided to reward these users with iTunes Store and Apple Store credits. Read on over here.