First, the probability that any portable nuclear devices were lost prior to or after the breakup of the Soviet Union appears low; the scenarios of loss offered by the special commission in 1996 are actually the least plausible among other possible scenarios. This does not mean that the threat does not exist, but rather that at this moment, it is probably not the most immediate threat to the home security of the United States or to U.S. armed forces abroad.
Second, even if any devices were lost, their effectiveness should be very low or maybe even non-existent, especially if the loss occurred during the period of the greatest risk, in the early 1990s. Without scheduled maintenance, these devices apparently can produce only minimal yield and eventually possibly no yield at all, and can only serve as a source of small amounts of weapons-grade fissile materials.
True "suitcase" nukes has indeed, very small amounts of fissile material in them and are reliant on injected 'hot' neutron sources such as tritium which 1. decay fast, 2. must be replaced, and 3. tend to contaminate other components - hence, regular maintenance is needed for them.
However.
This is only the case of the nukes made in the last 30 years.
The ones most worrisome are those left over from clandestine Soviet infiltration of key locations in the continental US during the 1950s and early 60's. They did this to ensure certain cities were hit even if their bombers were shot down.
Those were bulky, low-yield, and were very robust (lots of fissile material). These are the dangerous ones. One of these kinetic trigger types would make 10 high-yield nukes due to their poor design. But leave them sitting a few decades and you know what? You still get a mushroom cloud - but it's filled with tons of really dirty crap that'll poison the land for centuries.
And for those saying that OBL wouldn't use nukes, well, he had people slit women's throats and kill innocents - so how much can you actually say is beyond him? They will use nukes when they can get one, and it will be as soon as possible.