DaB has written a version in Java, and I have started one in C++/wxWidgets. I put mine into SVN, "yawr" (Yet Another Wikipedia Reader). Article browsing with images works quite well already, and I'm working on (fulltext!) search, some special pages, and a zillion other things.
There is also a reader written in C++ using the small C++-application server tntnet. It can already read and display the zeno file format (served via HTTP) and will be able to use the Wikimedia XML dumps soon.
Currently there are some developers working on a port of tntnet to Windows (using the poco classes to make it platform independent). As soon as the ready will be ready I will enhance it with a little dock application which will make it usable on a desktop system (as a HTTP daemon tntnet is now just a plain console-based application).