PmWiki has many skins. Some that I like: Grease Skin and Lean Skin. The ME112 09-10 wiki actually uses Light Skin, but looks more like the current version of Lean Skin. The ME310 09-10 wiki uses a custom skin titled "310intraskin" which would not be hard to customize further.
For a quick look at what's possible, see some of the ME310 team pages: Me310 Autodesk -- other pages available in right sidebar.
PmWiki has 2 kinds of authentication: AuthUser and UserAuth (default). The ME112 and ME310 wikis used AuthUser, which gives more control in setting up permissions for groups of pages.