BitTorrent 6.0, the official mainline client based on uTorrent, was released on July 28, 2006. Unlike previous official releases, there was no corresponding source code. While it's understandable that uTorrent's code may not be released, concerns remain that the protocol may advance while leaving the open source community behind.
"Sorry, source code for BitTorrent 6.0, like the source code for uTorrent, will not be released. However, versions 5 and earlier were of course released under open source licenses, and remain available for you to modify and redistribute subject to the terms of their respective licenses."
There are two aspects to the BitTorrent open source debate - the protocol itself, and the client.
With previous versions (prior to 6.0), the protocol was just as open as the old mainline client. This has changed with the release of 6.0. Developers, community members and newcomers to the BitTorrent scene have expressed concern that keeping up with the latest protocol developments may be difficult, if not impossible.
However this will not be the case, as the protocol will continue to be maintained at BitTorrent.org.
The protocol will continue to be published on BitTorrent.org and will continue to be available for everyone who's interested to see. It will not be closed or require a special SDK license to obtain the latest specifications. Additionally, BitTorrent will continue to update the site with the latest specifications.