Since many theme clubs have recently released their themes with the GPL license, some people started discussing the possible outcomes from this choice. Alex King has published two posts about it this morning, and a discussion has started on WordPress Tavern.
Go have a look, these are really interesting discussions, trying to draw the line between what is allowed, what is fair, and about WordPress ethics. It is also the occasion to get the opinion from famous WordPress developers such as Adii, Justin Tadlock, Cory Miller or Brian Gardner.