The New Republic's Yossi Klein Halevi has this from Shabtai Shavit, former head of the Mossad:
"My assessment is that, after they decided to aim for nuclear weapons, they advanced on three parallel tracks: enriching uranium, creating components for a bomb, and developing missiles. The missiles are ready for operation. As for enrichment, they have encountered all kinds of problems, like exploding centrifuges. I estimate that they made great progress, and very quickly, on the military track. Since they have problems with the uranium enrichment track, they can allow themselves to delay the military track, and wait for progress with uranium." [Halevi adds] Given that world attention has been focused on the military track, a tactical Iranian concession made sense.
This makes the Occam's Razor cut. It's the explanation that requires as few assumptions as possible -- in marked contrast to the NIE's judgement that Iran's nuclear activities are guided by a 'cost-benefit approach.' As another of Halevi's sources asks sarcastically, is it "a cost-benefit approach for one of the world's largest oil exporters to risk international sanctions and economic ruin for the sake of a peaceful nuclear program?"
One might respond that the perceived benefit is not energy but prestige, and risk management (keeping up with the Husseins), and keeping options open, i.e. getting 20 years of preparation out of the way so that a bomb can be produced at short notice. But the fact remains that getting the uranium enriched is the hardest part -- and that effort is going full steam ahead.