Cross-posted from the main development blog:
The plugin directory’s licensing guidelines have been updated. The guidelines will now allow code that is licensed under (or compatible with) version 3 of the GPL.
The guidelines still encourage use of “GPLv2 or later,” the same license as WordPress. However, we understand that many open source libraries use other licenses that are nonetheless compatible, such as GPLv2 only, GPLv3, and Apache 2.0.
Now may be a good time for plugin authors to review their plugins to ensure a license is specified. You can add License and License URI headers to readme.txt and the plugin’s headers. (You may also wish to include a copying permission statement.) For example:
License: GPLv2 or later License URI: http://www.gnu.org/licenses/gpl-2.0.html
You can see this used in the sample readme.txt.
This change brings the guidelines in line with the themes directory, which has for some time accepted GPLv3-compatible code. (Probably a good time to note that Creative Commons licenses are still incompatible with the GPL, and the theme and plugin directories.)