Um... I don't believe too many people on this site believe that this threat is a joke.
At one time, earlier on...everyone on this site had extremely staunch, conservative views and beliefs. Today, with the weak kneed (aka; pansy arsed) moderators in place...this site ain't what it used to be and doesn't carry the water that it once did. < editorial opinion/off>
The threat isn't a joke, but Paul L. Williams is a joke. He and Hamid Mir have been peddling the "Osama has Russian suitcase nukes" story for years. If Osama had them, he would use them.
The real threat comes from Pakistan and Iran, not old Russian weapons. It wouldn't take much for a rougue Pakistani scientist (see A.Q. Khan) to get the material and know how into the hands of terrorists.
Russia and China want global power, not national suicide -- they will push the US, but will generally avoid going over the brink. If Osama had a Russian weapon, the Russians would be working to get it back -- neither the Russians or the Chinese want one of their weapons used in the US.
On the other hand, Islamic extremists in the middle east are ready to die and perfectly willing to allow all of their countrymen to die in the process.