S5 v1.1 and later have been explicitly placed into the Public Domain by the original author, Eric A. Meyer. The following excerpt from the S5 FAQ spells out the details:
What are the licensing terms, and how do they affect the content of a slide show done in S5?
First, whatever license S5 has or did have, it will only ever apply to the technology, not the content. You can prepare a confidential presentation where the content is not only copyrighted and patented, but Top Secret as well. S5's license will not affect it.
As of version 1.1, the technology itself has been explicitly released into the Public Domain, so there are no restrictions whatsoever on its use or reuse (nor can there ever be). S5 v1.0 was published under a Creative Commons Attribution-ShareAlike 2.0 license, which still holds for that version of the software. The change was made because I found out CC licenses aren't appropriate for software only after 1.0 came out. Oops.
As a programmer, and not a copyright lawyer, this is merely an informed opinion. Please keep that in mind.
Under current copyright law, most code is considered to have copyright unless an explicit grant is made to the Public Domain. This makes a shared Public Domain project difficult, because it means that every contribution must contain a similar grant, as well as an implied warranty that the contributor owns the code (no borrowing from other projects, even open-source ones!) and has the legal right to donate the code into the Public Domain.
This creates a tracking problem for derivative works, especially where small patches and incremental improvements are concerned. A grant to the Public Domain can never be revoked, but there may be a loophole that will allow the community to grow without administrative headaches.
Creative changes to a public domain work may be entitled to copyright protection. Therefore, in order to grow the community around the S5 codebase and encourage sharing, this project is adopting the GPL for code which is not part of the original public domain grant. This project in no way asserts any copyright over the original work of Eric A. Meyer or other contributors to the original work placed into the Public Domain, but reserves the right to license derivative works under an alternative license.
Therefore, the portions of this program which are not part of the explicit public domain grant are licensed under the GPL, which should be construed to apply only to the portions of the code that may be considered a derivative work. For those portions, the following license applies:
Copyright 2010 Todd A. Jacobs, et al.
This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.
The idea is that patches, bug fixes, new features, and incremental improvements can be added to the project without having to make (or track) an explicit grant of any kind; authorial attribution by the submitter is all that should be required, and such tracking of committers is baked into the Git revision control system.