Scarily, though, some of them are very bright indeed.
Look at the well-planned and well-executed attacks on 11th September, 2001.
My personal sense is that if you're getting clicks on the geiger counter whilst parked up outside a mosque in, say, Denver, you're too late.
I can't imagine that al Qaeda would commit anything less than its best agents to an attack involving such hard-to-obtain and difficult-to-handle material.
A worrying prospect.