As of March 31, it is now official: MediaWiki has a new logo (see the official announcement here). The description for the logo, designed by Serhio Magpie, explains the basic idea:
The new logo represents a collection of projects built on our engine: each petal is one of the many wikis that we support, and the lack of an explicit core shows that we are part of these projects, as well as and they are part of MediaWiki. The new logo also reflects the fact that evolution never stops, and like the petals of a flower, the development of each project, the growth of each community built on our engine allows everyone else to grow.
The motivation for a new logo was that the old logo had not met the visual and technical requirements for a long time. The sunflower logo, as Erik Möller wrote in an email, had been quickly put together and certainly served well over the years, but “it basically violates all rules of good logo design.” It also almost became the official Wikpedia logo:
Fun fact: It was originally a submission to the Wikipedia logo contest, where it came in third and the current Wikimedia logo came in second. It was pretty close – in some parallel universe, the internationally recognized symbol of Wikipedia may not be the puzzle globe, but a sunflower surrounded by square brackets. Make of that what you will. ;-)
The new logo can now be used much better in large or small formats and it can be adapted for different use cases. So there are already numerous variants or discussions about variants here, here and here.
And finally: Helpful for anyone who wants to use or adapt the logo are these guidelines on mediawiki.org.