The office of president in Iran is rather unlike the office that goes by the same name in the United States. Ahmadinejad is not the commander-in-chief of the armed forces and he therefore may not have a whole lot of say in the fate of the British sailors held in Iran. Ahmadinejad may be a terrible person, but at least his power is limited under the Iranian constitution.
Moreover, it appears in an article that Juan Cole linked to that Iran is simply looking for the British to admit wrongdoing in the affair, i.e.: admit that they were in Iranian waters. It is also worth remembering that the waters in question have been subject to boundary disputes, and that makes me wonder whether the maps that are constantly put in the newspapers and on TV are of the Iraq territorial claims, or of the Iranian territorial claims. Are they some melange of both sets of territorial claims?
Of course, it may be that the British were indisputably in Iraqi waters, but the question of where the boundary is has been cast aside – perhaps in the hope that we’ll just start assuming that the British were correct. The Iranian offer to free the soldiers may be a ploy, but regardless, it’s interesting that that element of the story has been far less reported than images of Tony Blair looking indignant on TV.