Welcome here. This is a very good question !
We are working for a client with Cocktail, developing a proprietary framework with it. And they asked the same question last week.
It seems that the answer is yes, with the GPL, for a library like Cocktail. And this is a mistake in the choice of the license. It is due to the fact that our other open source projects (amfPHP and Silex) are software with a plugin system, and not libraries.
So we will change Cocktail license to allow the use of Cocktail for proprietary projects. I guess we will go for a MIT license.
Do you plan to use Cocktail soon? This would give us a reason to do the change quickly.