Check GNU GPL license and Creative Commons Attribution 2.5 license.

I will not be surprised if thousands of folks jump into this mess with their themes, obviously they are not aware of or care enough about the possible issues with using GPL. One way to overcome this would be to offer a no frills, striped down version with not advanced features and ask the folks to in turn download the advanced features (CC licensed) version from your site.

It worries me that Automattic are putting pressure on people to release under GPL without fully understanding the implications of doing so. I know that ultimately it’s the designer’s responsibility to check that they are releasing their work under an appropriate licence, but many don’t realise the importance of the issue until it bites them. I wish they would post a plain English outline of what GPL-compatibility entails, rather than simply directing people to a list of licences with no actual information; but of course being upfront with designers about what they were signing up to might deter them. Or, at least it would if they could be bothered to read it. In practice, people are going to upload linkware and non-commercial stuff and claim it’s GPL, probably in genuine ignorance that it isn’t, and then get angry when people rip their links off and replace them with spam.