after I saw this
thread I decided to opensource my launcher that I use in my game
It supports Windows, Linux and Mac OS X, it's defensively coded and uses reliable technique for updating. Tested on thousands of installations of my game without any bugreports other than non-usage of proxy (most likely Java's fault) and some problem with resolving hostname under linux (most likely external fault). Basically It just works [tm]
You can download the sources here: launcher-20110520.zip
(under ZLIB license)
You can test it live on my game, in download section
. You can see the descriptor used here: update.xml
Note that the launcher is totally unsupported (except questions about the code in this thread) and I don't intend to continue development of it. Feel free to fork it if you want to continue the development.