The problem probably is mime-type based.
First, aussure your webserver sends the right mime-type for jnlp files:Using apache add the follwing to the httpd.conf file:
AddType application/x-java-jnlp-file .JNLPIf you have no access to server-configuration files, but can use PHP or JSP, one of the follwong hacks should help:
1. PHP-based: Rename the .jnlp file to .php and add the following at the top:
<?php header("Content-type: application/x-java-jnlp-file");
echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>";?>2. JSP-Baed: Rename the .jnlp file to .jsp and add the following at the top:
<%@ page contentType="application/x-java-jnlp-file" %>Configure the MIME-Type on the client machine:This is done for IE automaticly (and also probably for MacOS X - I don't know

)
In other browsers like Mozilla or KDE's Konqueror you can set them manually in the preferences. Simply add
x-java-jnlp-file to the
application group using the .JNLP file asociation with the JavaWebstart executable (javaws[.exe]) in the $JRE_HOME/bin directory.
Firefox doens't offer this setup, but just select open with (instead of save as) and search for javaws[.exe] with the 'Do this automatically for files like this from now on' option checked.
Hope that helps
- Michael