Re: GPL Question re upcoming plugin release
You will have to license your work under some version of the GPL that at minimum offers the freedoms that the WP GPL offers.
What others have said above is basically correct. The GPL does not say anything about free as in cost. It is about freedoms of the end user. In fact, the GPL does not discourage developers or theme designers from making money off of their work or someone else’s work.
However, the GPL is very specific in how this is to be done: either by charging a distribution fee, a support fee, or both. So, in effect, you are not charging for the code itself. The vast majority of Premium plugin shops simply charge a distribution and support fee. If their plugins are not GPLed, then something is wrong.
Here are two great links from the GNU website that explain this very clearly:
By the way, IMO there should not be a double standard in the WordPress community. Premium theme designers abound. Premium plugin developers should also be afforded the same respect and opportunity. The crucial point is that their work, like premium themes, must remain licensed under the GPL.
Of course, as Matt Mullenweg has stated on numerous occasions when discussing the issue of premium themes, although the PHP must be GPLed, the CSS and JS of the theme can be copyrighted. The same holds true with plugins. But, in my opinion, doing so is just too picky. I prefer to GPL everything–the PHP, CSS, JS, etc.
Now, you could choose to license your work under the AGPL, which is considered to offer more freedoms to the community. The basic difference from the GPL is that any changes made to your plugin, even if for private use, must be returned back to the community. Under GPLv1 through GPLv3, it is acceptable for any code changes to a plugin to remain private if the plugin is never distributed.
One final thought. I’ve stated this before in other places. Many people in the WordPress community seem to be moving beyond the spirit of the GPL, to a more utopian, share and share alike vision. There’s nothing wrong with that if it happens. But the GPL has never been about free as in cost.