Via the Times of London:
Iran ‘to try Britons for espionage’-News-World-Middle East-TimesOnline

FIFTEEN British sailors and marines arrested by Iran’s Revolutionary Guards off the coast of Iraq may be charged with spying.

A website run by associates of Mahmoud Ahmadinejad, the Iranian president, reported last night that the Britons would be put before a court and indicted.

Referring to them as “insurgents”, the site concluded: “If it is proven that they deliberately entered Iranian territory, they will be charged with espionage. If that is proven, they can expect a very serious penalty since according to Iranian law, espionage is one of the most serious offences.”

The warning followed claims by Iranian officials that the British navy personnel had been taken to Tehran, the capital, to explain their “aggressive action” in entering Iranian waters. British officials insist the servicemen were in Iraqi waters when they were held.

At this point one of the more important words in the above paragraphs is “may” as in they may be tried for espionage. My guess is that ultimately they will not be tried, as I think that Iran is trying to leverage this situation towards some goal and wishes to turn up the heat as much as possible in an attempt to achieve that goal. Precisely what that goal is remains to be seen. It could be securing the release of Iranians captured in Iraq, or it could be an attempt to force some sort of talks on the sanctions passed this week. There is also the clear usage of such prisoners as tokens for internal political consumption as evidence that the current regime can stand up to he powers from the West.

My guess is that it is a combination of the domestic politics angle, the prisoner swap scenario and anger over UN sanctions. The piece notes:

Iranian student groups called yesterday for the 15 detainees to be held until US forces released five Revolutionary Guards captured in Iraq earlier this year.

Al-Sharq al-Awsat, a Saudi-owned newspaper based in London, quoted an Iranian military source as saying that the aim was to trade the Royal Marines and sailors for these Guards.

The claim was backed by other sources in Tehran. “As soon as the corps’s five members are released, the Britons can go home,” said one source close to the Guards.

He said the tactic had been approved by Ayatollah Khamenei, Iran’s supreme leader, who warned last week that Tehran would take “illegal actions” if necessary to maintain its right to develop a nuclear programme.

And, I suppose, we can’t rule out a fit of pique by high ranking officials over recent events:

Intelligence sources said any advance order for the arrests was likely to have come from Major-General Yahya Rahim Safavi, the commander of the Revolutionary Guards.

[…]

Safavi is known to be furious about the recent defections to the West of three senior Guards officers, including a general, and the effect of UN sanctions on his own finances.