Seeing as the subject has been brought up, copied from the GPL linked to by Andrea_r, I believe hurricane is referring to
You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty;
Conspiciously means noticeable. I don't think a comment hidden in the code is noticeable to about 95%+ of the users of the plugin? But that aside, there's no actual disclaimer of warranty in there either. I've always had one on my website's plugins section and more recently added it directly into the plugin, but as per WordPress's own plugin restrictions page, it comes under GPL v2 if there is no license present.
You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.
I don't believe that's there?
I'm still running WP 3.2.1 and it seems that the upgrade to AIOSEO 188.8.131.52 has broken the Menu Label functionality on my site
This is because a vital part of the code has been removed during the copy and as your theme uses span tags in the menu it's breaking it.