Via Schneier, excellent article in Legal Affairs comparing modern terrorists with pirates. Parallels are strong, complete with states backing pirates for deniable attacks on other states, and pirates seizing land and forming mini-states in wild lands. The emphasis is on the law of the sea as precedent for an international legal framework for dealing with terrorism.
One difference, which is significant to the argument that the threat of terrorism is exaggerated: piracy was essentially for-profit, and therefore sustainable. Terrorism creates occasional opportunities for plunder, but is generally a loss-making activity requiring external funding. Therefore it is less likely to be as widespread and as near-permanent a problem as piracy was (and is).
Where terrorism is profitable, it is in "danger" of decaying into pure gansterism, losing its political side where that is not good for business. I believe that happened to a certain extent in Northern Ireland.