This is mostly a question for Erik Moeller and Lee Daniel Crocker. The new README in CVS says,
Sections of code written exclusively by Lee Crocker or Erik Moeller are also released into the public domain, wich does not impair the obligations of users under the GPL for use of the whole code or other sections thereof.
First, I'm pretty sure you can't release something under the GPL and also disclaim copyright to it.
Second, it's not clear that you can contribute to a GPL project like Mediawiki and make "parts" of that work public domain. What parts of your work are not derivative of previous work, and thus not required to be under the GPL? Remember, derivation doesn't just mean "containing parts of previous stuff" -- adding a function to an existing module is making a derivative work (the new module), even if "your" function doesn't contain any code from the other modules.
Lastly, who benefits from this? Why make this confusion for users? How am I supposed to figure out what parts of MediaWiki aren't encumbered by the GPL? If it's pretty much impossible to me to find out, what good does it do? And how does that outweigh the downside of confusion to the 99% of other users who don't need the PD dedication?
If somebody wants to put parts of MediaWiki under a separate license, they should be really clear about what's under that separate license, and they should make real sure that they aren't violating the GPL by doing so.
~ESP