The press release barely mentions it but it's true. The free private repos are cut down, including bizarre hold-backs like wikis. GitHub's wikis aren't great, but why hold them back? It's kind of mind boggling.
Public repo wikis would get read by an arbitrary number of people. Private repo wikis would only get read by the 1 to 3 repo collaborators. So perhaps they thought it wouldn't be a big deal to disallow them.
