[Skybird]

Der Spiegel's view on it reminds of some interesting historical details:

http://www.spiegel.de/international/...474518,00.html

Most of Iran's oil wealth lies concentrated in Chusistan province, which is why the British would have liked nothing more, after World War One, than to make that stretch of land with its Arab population part of a British-controlled sheikhdom. But that was prevented by Shah Reza Pahlevi, who managed to consolidate his power. Still the region remained disputed, because the British remaining in Iraq continued to covet it.
Violating international custom, the British fixed the border along Shat al-Arab in such a way that the entire river, which marks the border between Iran and Iraq, became Iraqi territory - right up to the Iranian coast. It was only in 1975 that the government in Baghdad accepted shifting the border to the center of the river - a concession in return for which Shah Resa Pahlevi ceased supporting insurgent Iraqi Kurds.
In 1980, Saddam Hussein changed his mind, and the eight-year war between Iraq and Iran began with an Iraqi bombardment of the Iranian oil refinery town Abadan on the eastern bank of the Shat. Britain and the United States sided with the Iraqi dictator, providing him with military reconnaissance, weapons and even poison gas - a decision that continues to represent a bitter legacy liability for the West, and especially Britain, to this day.
Andrew Phillips, a British member of parliament, recently noted that the number of Iranians killed between 1980 and 1988 is comparable to that of British losses during World War One. In Iran, anti-British sentiment isn't limited to conservatives or to the radicals surrounding President Mahmoud Ahmadinejad. It's much more deep-seated than the hatred of the "Great Satan," the United States, that is constantly reiterated, partly for propaganda purposes.

(...)

But Tehran could also try exchanging the 15 British prisoners for the Iranians arrested during US raids in Iraq in the past months. This strategy would at least have a diplomatic framework and a forseeable schedule: Representatives of Iraq's neighboring countries and of the United States and Britain want to meet in mid-April in Istanbul for a second conference on Iraq, and this time the countries' foreign ministers will be taking part.
So the British sailors are probably facing a two-week wait - provided careless statements don't further escalate the situation.

Not so much an excuse of Iran's action, but maybe part of the explanation. there are no single, isolated events. Only events that are embedded into greater contexts.