DavidR,
This is a new security policy that is going to be brought in inside Firefox and also IE and it is both run by servers and browsers to check on each other what is allowed to run there security wise. I have it as a security add-on inside Firefox. Firefox 3.6 Beta has it built in.
I hope both browser developers and web developers are going to implement it,
It aims at unpluging scripting attacks, meant for those that cannot work NS to its full potential,
polonus