I’ve set up an installation of Mediawiki to play with. Here’s a link.
Mediawiki 1.4.0 was just released earlier this week and there appears to be a problem with the install file. I had to copy a file out of the 1.4rc1 archive to get the installer to work, and now the default skin looks somewhat broken. I’m going to hack around and try to fix it.
Update: Looks like I fixed the problem, more or less.