Licenses on patches / hacks to GPL'd software

[Andrew van der Stock]

Hi there,

The software package I work on most actively at the moment is about to move
to the GPL. The problem starts with mod / hack groups who have developed
additions to the software. Most of these places have instituted licenses
which I believe are incompatible with the GPL due to further use
restrictions. 

I will give several examples:

* a simple patch which removes a feature from the main software 

* a complex patch to change to the look of the main software (new skinning
engine)

* a complex addition which changes the main software, changes the database
schema, and adds new original files to the main software 

I believe the first to be a derived work. The other two? What is the usual
position in relation to licenses for patches and the GPL? 

Thanks,
Andrew