I had not put much thought into that question, assuming that between the troops in boats in the Persian Gulf and the troops in Afghanistan, Iran doesn't need vulnerable US troops in Iraq as sufficient deterrence from an unprovoked US strike.
But Juan Cole points me to another factor that I had not considered. There are still thousands of Americans working officially in Iraq in the Green Zone after the last combat troop and trainer is gone.
Even the US embassy in Baghdad would be vulnerable to massive attack, especially once the troops are out. Al-Maliki supported Lebanon’s Hizbullah against Israel during the 2006 war, and would certainly adopt the same position in the event of another conflict, kicked off by a bombing of the Natanz facility.An Iranian response to a US attack could probably kill almost as many US personnel in action after the US leaves Iraq as it could kill there now. The political effect of a sustained campaign against US forces on the US' political desire to remain in the region could be just as profound in a post-exit Iraq.
In a very real way, nothing changes with the US pulling troops out of Iraq other than the US saving money on the costs of keeping them there and the US slowly losing leverage over the Iraqi government.
It is a good thing, and I am glad for many reasons to see the US troops leave. But it does not substantially change the strategic situation one way or another.