The Obama administration is sending mixed signals to Iran.
Obama's decision to back off Syria strikes, and I'm bolding this part because it's important, boosts the credibility of his stated position that he isn't seeking Iran's destruction and that he will seek peace with Iran if they first meet his long-held demands on nuclear enrichment. That's exactly the message Tehran needs to hear right now.
I think we need be careful not to get too carried away. The Russian plan has barely been put to the test. We don't know if the Assad regime is genuinely willing to hand over its chemical arsenal or whether they're simply going to stall for time and hope that the political will to launch punitive strikes further erodes in Washington. If Assad is simply stalling and manages to avoid military strikes without surrendering his arsenal, Tehran will likely draw a very different lesson than the one Drezner and Fisher think they're currently receiving.
Moreover, it's going to be very difficult for Iran to accept the idea that the Syrian deal shows the Obama administration isn't seeking Iran's destruction when the Pentagon talks openly about arming Syria's opposition even with a chemical weapons deal in place. That sends exactly the opposite message to Iran, who need only look to Libya to understand the consequences of accepting a Western disarmament deal.
Finally, it's also worth considering what lesson Washington will take away from this: namely, that threats of military force are vital to forging a diplomatic breakthrough (something many Iran analysts have been arguing for a long time). If this becomes the conventional wisdom, it could provoke the administration into another high-wire act, threatening military strikes against Iran and then banking on a last minute diplomatic breakthrough to peaceably bring about a deal.