[shawger]

What changes would you be making to the GPL? There's probably another open source license that would fit your wishes better. You also mentioned something about using punBB. Keep in mind that punBB is licensed under the GPL, so if you are using punBB code then your code will have to be licensed under the GPL as well.

As far as interest for this type of thing goes, there's already a ton of open source CMSs. That being said, more code is always better and at the very least people could see what you did and try to adapt it into their own website even if they weren't using your CMS. I'd say go for it. Opening up your code so people can look at it and see the cool things you did is never a bad thing.

The other thing that could happen is that other people will be able to help you with development. If you plan on using this CMS again for your website then you might find yourself with a couple more features than you would have had.