Why haven't we been hit by a dirty bomb?

From LittleGreenFootballs: "Slovakian police arrested three people trying to sell an undisclosed type of radioactive material for $1 million. The Times of London has more news, and it isn’t good; they weren’t just selling discarded Timex watch faces, it was almost 500 grams of highly enriched uranium in powdered form."

It would seem that there is lots of radioactive material around, and attacking a Western country with a bomb filled with radioactive material would have a tremendous psychological impact. So why hasn't al-Qaeda attacked us with a dirty bomb. It can't possibly be because our border security is too good to allow terrorists to smuggle in a bomb. I suspect, rather, that al-Qaeda fears how we would respond to such an attack.