I think the point is that the sunflower logo was one which we wanted to have as the default logo for all installations, and we wanted people to be able to happily use it or do whatever they wanted with it, and we did NOT intend to protect it or license it in any restrictive way. Not that particular one. That's because it is the default logo in the default install of the software, see.
This is not about the foundation logo, the wikipedia logo, etc., all of which are trademarks and copyright to the foundation, etc.
The sunflower one is one we don't really care about, except of course we love it with all our hearts. :)
Now, as to whether it is literally "public domain" (which is really rather a bit hard to do under US law), I leave to lawyers to figure out. Whether it is a trademark that the Foundation chooses not to defend, or a trademark which the Foundation freely licenses, or not a trademark at all, is a question I also leave to lawyers to figure out.
The point is, people can do as they like with it, and the Foundation does not mind.
This discussion, to repeat, applies ONLY to the sunflower logo which is the default install in the mediawiki software.
--Jimbo