The world has generally been confused by Iran's actions in taking British marines hostage. Why, after so nearly becoming engaged in a war with the U.S., would Iran choose to engage the U.S.'s number one ally so aggressively?
Here's an idea. The Iranian government is trying to get the new Iraq leaders used to the watery boundaries that the Iranian government wants. The Iranians are demonstrating clearly the boundary that will trigger a war and the boundary that they are willing to fight for.
After a time has passed, the hostages will be released. The message would have been conveyed to the Iraqis "Don't let your people go here or it will be considered close to an act of war."
What the Iranians may not be considering is that dramatic actions like these take on a life of their own. Once you do something like take hostages, the outcome cannot easily be controlled. Iran has seriously placed itself in a position to get attacked, this time led by Britain and supported by the United States. What the hell are they thinking?