The problem can be reproduced each time, if you have both 1.6.0 and 1.5.0 installed.
If only 1.5.0 installed (1.5.0_11 through 1.5.0_16), there is No problem.
If only 1.6.0 installed there is No problem.

1) save the two attachments on a PC
2) modify singleapp.jnlp to point to helloSinged.jar
                  codebase='file:///C:\Documents and Settings\user\Desktop'>
3) double click on the JNLP file
4) notice the exception

java.lang.ClassNotFoundException: HelloWorld
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at com.sun.jnlp.JNLPClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at com.sun.javaws.Launcher.doLaunchApp(Unknown Source)
at com.sun.javaws.Launcher.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

Customer is experiencing this on Webserver accessing the JNLP file via a Servlet.
We took out the Servlet out of the picture to make it easy to reproduce and avoid deploying it on a Web/App server here.

From ###@###.###:
This is a bug in our caching and I will look into where it comes from.