A number of high level nuclear experts who know something about suitcase nuclear technology have indicated that any such devices are no longer operational. These devices require constant maintenance (the radiation from the payload disintegrates the control circuitry). Also, these devices are very noisy radioactive sources that can be readily detected by nuclear sensors.
Last point, Al Quada is mostly a bunch of savages with little technical knowledge. They are also opportunistic. If they had a chance at this level of attack, it would have been done long back, while they still had the chance.
The whole story line makes for a good book, but I won't be reading it.
In addition, bin Laden has recruited former Soviet scientists and technicians to maintain these weapons and recharge their nuclear cores so that they may be deployed immediately on his command.
Only very very very slowly, if at all. The amount of shielding needed to keep electronics safe from, for example, alpha particles given off by plutonium, is very small - perhaps nothing more than the normal package on an IC is needed. Your skin stops alpha particles.
Simple nukes that do not use tritium to enhance yields have a long shelf life. The reccomended service interval might be (guessing) 6-18 months or so, but they should be "good" for many years, maybe 20 or more, depending mainly on how batteries are stored, or if they can be installed just before arming them, etc.
Underestimating your enemy is the beginning of your end.
If you believe that Al Quaida consists primarily of those individuals hiding in Afghanistan and Pakistan caves then you really are naive.
I suggest you visit your local mosques or those in Seattle, Detroit, Chicago etc and you'll see your neighborhood Al Quaida representatives that are not only well educated but also well off financially.
Why does everyone assume these alleged suit-case nukes are old? Who is to say they havent been built in the last 5 year?
In your opinion, will we get hit next with a Bio, Chem, Dirty bomb or Nuke?