This is a little embarrassing for how simple it is, but I found the way to bypass the signing issues (which doesn't require a VM restart

)
Just execute this code early on in your main class. It installs a dummy security manager. Then all the webstart security crap is bypassed (but to get to the stage where your Main class executes you'll still need to sign your jars

)
//grant all permissions on the clientside
Policy.setPolicy( new Policy() {
public PermissionCollection getPermissions(CodeSource codesource) {
Permissions perms = new Permissions();
perms.add(new AllPermission());
return(perms);
}
public void refresh(){
}
});
Of course this requires the 'all-permissions' security dialog to be accepted.
I found it here:
http://forum.java.sun.com/thread.jspa?messageID=3755247 
Keith
PS: Another interesting webstarty fact I learned today was that 1.4+ doesn't mean "get the latest JVM which is at least 1.4", it means 1.4.* is preferred over 1.5 and anything else. So you need to put in loads of tags like this to get the latest JRE, but one which is greater than 1.4. See here:
http://mindprod.com/jgloss/javawebstart.html<resources>
<!-- Acceptable JVMs in preferred order, best first -->
<j2se version="1.6.0" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.6+" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.6.0" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5.0_09" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5.0_08" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5.0_07" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5.0_06" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5.0_05" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5.0_04" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5.0_03" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5.0_02" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5.0" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.5+" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_13" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_12" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_11" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_10" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_09" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_08" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_07" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_06" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_05" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_04" href="
http://java.sun.com/products/autodl/j2se" initial-heap-size="128m" max-heap-size="512m" />
<j2se version="1.4.2_03" href="
http://java.sun.com/products/autodl/j2se" />
<j2se version="1.4.2_02" href="
http://java.sun.com/products/autodl/j2se" />
<j2se version="1.4.2_01" href="
http://java.sun.com/products/autodl/j2se" />
<!-- 1.4.2+ means any 1.4.2 version not containing a dash e.g. not 1.4.2-beta -->
<j2se version="1.4.2+" href="
http://java.sun.com/products/autodl/j2se" />