Ahmadinejad is really a figure head. The real power lies with the Assembly of Experts and Supreme Leader Ali Khamenei who replaced Khomeini. There are many things swirling about Iran these days including UN sanctions, nuclear weapons development, and domestic unrest due to a failing economy and corruption. The seizure of the UK sailors may have been for domestic consumption and to placate the powerful Revolutionary Gurad who are unhappy about the detention of their compatriots in Iraq. It is also interesting that since the seizure, the price of oil has gone up bringing in more oil revenue to the Iranian coffers.
Thus, we should be very careful as to how we react.
I am not suggesting to go and blast them, I am saying that we should be open to this being a bigger issue than it might otherwise seem.
Pukin Dog raises an excellent point that this may be due to the vessel that the British intercepted.
BTW in prior posts, I think that I was very clear that I'm not advocating invading Iraq-- quite the contrary. However, there's no call at this point to pull back from what's going on in Iraq.