Always read the terms provided with plugins and themes carefully. They may not be entirely covered by the GPL.

In particular plugins are expected to offer GPL terms for the html and php code but are not obligated to provide the CSS, images and Javascript under GPL terms. See the following article.

An example of this would be the S2MemberPro plugin. See the S2MemberPro terms.

Another example would be any theme or plug-in purchased through Envato where the author-provided license does not extend GPL cover to all the provided files. The Envato default license considers the CSS, JavaScript and images to NOT be covered by GPL unless the author-provided license says otherwise. See license terms and split license terms.

I am a supporter of Jeff's proposal for theme / plug-in authors to clearly state what their GPL license covers. In particular whether the GPL license applies to ALL the supplied files including css, images and JavaScript.