We know that there are many forces at work that would like to detonate a nuclear device in Manhattan. We know that there is a black market for nuclear materials. We know that such a catastrophe would make 9/11 look like child's play—this piece, for example, estimates more than 800,000 deaths from a 150-kiloton nuclear bomb (the size of a US nuclear cruise missile) detonated in Times Square. (These things always assume the bomb will detonate in Times Square. It is a fair assumption. Even New York City residents often dream of detonating a bomb in Times Square.)
We know it would be bad. What we don't know is how likely such an attack really is. Is it a total impossibility? A faint outlier of a chance? Or a near inevitability, given the extent of our enemies in the world? Your answer will depend on your knowledge of geopolitics, your personal views on human nature, and your general confidence in the accuracy of your own uninformed opinions.
I put the chances at 68% at some point in my lifetime.
Put your own estimates in the discussion section below. When it happens, none of your guesses will matter.