I have tested XPontus under JDK 1.6 on Windows and Linux. If you have issues delete the application from Java Web Start applications manager(run javaws -viewer). I have had some strange problems which might be related to Java Web Start application caching.
If you have XPontus installed make a backup of your hidden folder .xpontus(in the directory mentionned below).
Extract the file plugins.zip in your user home directory /home/myuser on Linux/Unix and C:\Documents and Settings\myusername on Windows.