From August 4th all WordPress themes on theme forest will have to distribute their php code containing WordPress code under the General Private Licence (GPL).
The GPL licence means everyone can modify and redistribute the code freely, as long as they give the source code with it. In WordPress the source code is the code, so when giving the php files you give away the source code as well.
As Theme Forest stress it, GPL is not evil. It is the foundation of free software and without it you wouldn’t be able to use this fantastic platform that is WordPress. Now we understand and share the concern of all WordPress themes developer but it is just the way it is. You will have people buying your themes and then reselling some of your code. But bare in mind that most people buying themes a probably not tech savy and will not always take advantage of the code the way they could.
People are lazy and that wont change.
Here is the link to the original post on the WordPress blog: http://wordpress.org/development/2009/07/themes-are-gpl-too/