A senior wonderland enginer reports :
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A couple of brave community members have tried running, via webstart, Project Wonderland (http://wonderland.dev.java.net) with JDK 6u10 and both have run into the following exception

Caused by: java.lang.SecurityException: sealing violation: can't seal package javax.media.jai.operator: already loaded
    at java.net.URLClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.access$000(Unknown Source)
    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)


This works fine under other versions of JDK 6 and JDK 5.

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I can verify this is so starting with b21 that this webstarted app fails.
 
That build contains on;ly deployment fixes - a large number of them so
I'm not sure what the cause is.