Future released versions of WordPress will be a bit more clear about how WordPress is licensed. Specifically, it will be clarified as licensed “GPL version 2 or later” instead of its current licensing, which is GPL with no version specified. Mark Jaquith went into detail about the slight, but important change, on the WordPress development blog:
However, WordPress contains libraries which are licensed under the GPL “version 2 or any later version,” which obviously excludes version 1 of the GPL. Here is the reality: the GPL version 1 is effectively irrelevant. It hasn’t been a commonly used license since before Matt Mullenweg was in third grade! Clarifying WordPress as being licensed under the GPL “version 2 or later” resolves these niggling library licensing concerns or ambiguities, and clarifies where WordPress stands.
It was the intention of the WordPress founders and developers to be GPL version 2 or later from the beginning, and we have now made that license properly explicit.
When WordPress is listed under an unspecified GPL version, it means that it can be distributed under any version of the GPL. As Jaquith explains, that complicates situations where libraries bundled with WordPress have a specific GPL version listed, as they can not be distributed under any GPL license. The change to more GPL version specificity is a simple one, since while switching to a non-GPL license would require relicensing, choosing a subset of GPL versions to distribute does not.
On a related note, plugins will still only be accepted into the WordPress.org plugin directory if they are licensed under an unspecified GPL version, or GPL version 2 or later. Jaquith explains the restriction in the comments of the post:
If we allow GPLv3 or GPLv3+ plugins into the repository, it makes it tricky to bring those into WordPress core at some point.
Licensing hats off now: this change, while important, will most likely not effect you much. Go hug somebody and tell them about WordPress.