Suitcase nukes are the stuff of yellow journalism and trashy spy novels. They would have to b manufactured by a sophisticated state, of which the US and the Soviet Union alone had the technology to build. The Soviet Union's security struggles in the mid-to-early 1990's would be the only opportunity to steal them. These things, however, would require extensive maintenance - which Al Queda and Iraq could not provide.
Suitcase nukes, therefore, are not a credible threat. (Source)
Ok how many of this sized nuclear bomb (1950's technology) can you fit in a shipping container?
This technology is in the hands of China, North Korea, Pakistan, India, France, Great Britan, Russia and most of its now independent former republics, Israel, and probably several others. It will soon be in the hands of Iran.
They do not need to launch a missle from Tehran to strike America. All they need to do is to drop one in the hands of some independent terrorist group and it will be on the next boat to New York Harbor. They will have plausible deniability and will point their fingers at Israel and Pakistan.
Suitcase nukes are simply small warheads.
Other nuke options exist including various dirty bomb and radiation poisoning schemes.
The major options would always have been chem/bio. Bio was the most formidable, and the reason all the troops were required (initially) to get the anthrax shot.