Reading the diplomatic leaves on Iran

On the eve of the Security Council coming to a deal on Iran sanctions, the Iranians choose to detain 15 British soldiers.

(It's still not clear whether they were in Iranian waters or not, we'll probably never know, but they had likely been to that exact same spot several times before, and the Iranians chose this moment to grab them.)

Now, after lobbying so hard for a huge number of visas so that Ahmadinejad, security, and staff could come to the UN to plead their case against the sanctions, Ahmadinejad suddenly pulls out saying he won't address the Security Council.

So, what's the play here? Feeling a loss of protection from Russia and China, the Iranians detain the British to send a more direct threat/message of Iran's potential to complicate Iraq?

I don't see how this works, but it's definitely a ratcheting up.