At last in the British media, I find a reference (in The Daily Record) to what probably sparked and lies behind this whole episode: "Some in Iran have called for the 15 to be held until the release of Iranian diplomats and at least five Iranians detained by US forces in Iraq for allegedly being part of a Revolutionary Guard force there."
EVENING UPDATE: Some have asked for more on this story. Conveniently, Pepe Escobar has just provided it at Asia Times Online. You can find news of the original arrest of the Iranians at Reuters. Pepe Escobar concludes:
Tactically, as a backgammon or, better yet, chess move - in which Iranians excel - the Shatt-al-Arab incident may be much more clever than it appears. Oil is establishing itself well above US$60 a barrel as a result of the incident, and that's good for Iran. It's true that from London's point of view, the incident could have been arranged as a provocation, part of a mischievous plan to escalate the conflict with Iran and turn Western and possibly world public opinion against the regime.
But from Tehran's point of view, for all purposes British Prime Minister Tony Blair is a soft target. The episode has the potential to paralyze both President George W Bush and Blair. Neither can use the incident to start a war with Iran, although Blair has warned that his government is prepared to move to "a different phase" if Iran does not quickly release the sailors.
If the Tehran leadership decides to drag out the proceedings, the Shi'ites in southern Iraq, already exasperated by the British (as they were in the 1920s), may take the hint and accelerate a confrontation. Strands of the Shi'ite resistance may start merging with strands of the Sunni resistance (that's what Shi'ite cleric Muqtada al-Sadr has wanted all along). And this would prove once again that you don't need nuclear weapons when you excel at playing chess.