...and while the "boomy" bombs are on the risk assessment grid, the dirty bomb scenario might still be easier to pull off, offeres up a similar amount of terror payback and fits in better with the current strategies of threat.
The scenario that takes place in "Dirty War", a 2004 HBO movie from a British production company, is hard to find fault with in terms of realistic implications of a dirty bomb being detonated in most any urban area.

http://en.wikipedia.org/wiki/Dirty_War_(film)

It should be mandatory viewing for first responders, and certainly opens my eyes to what my spouse might get caught up in working downtown in her "known terrorist targeted" highrise.

Also available on Netflix:
http://movies.netflix.com/WiMovie/Dirty_War/70023523?trkid=2361637#height1834

Even the reviews consider it a worthwhile view.

http://en.wikipedia.org/wiki/Dirty_War_(film)

"L.A. dry run shows urban nuke attack 'a survivable event'"

http://content.usatoday.net/dist/custom/...=41773400.story

There is this great animation show shrinking fallout zone:

http://www.usatoday.com/video/index.html#/Fallout+from+an+improvised+nuclear+device/712474766001

"The government would be preoccupied with so many tasks — identifying the culprit; tracking the effects of the blast; securing government buildings; ushering critical personnel to safe locations; amassing drugs and supplies — that federal help won't arrive for 24 to 72 hours, the White House guidance says."