I have heard a lot of talk (not on this thread I hasten to add) that Paul Williams is just trying to sell books and he shouldn't be believed because he could have financial motives for saying this stuff. I really don't think that this is the case though.
I have read his books before and I would not say he is a sensationalist. His information also concurs with Yossef Bodansky's book "Bin Laden, The man who declared war on America", which was released in 2001 before 9/11. The fact that Bin Laden bought suitcase nukes has been out there for some time. He's not the only one saying it.