Reading the comments here are so damn funny.
They think that Putin is poking Obama in the eye because he's just so damn weak.
When in reality, Putin has already agreed to our terms that he wouldn't sell missiles to Iran, and as far as we know he hasn't at all. And now, rather than saying "**** obama, we're trading this", instead he says "once Iran has negotiated a deal to allow inspectors in to the country and to cooperate with everyone, we will sell them defensive systems."
This is essentially "once Obama and them says it's ok we can start trading."
If the deal means that Iran allows constant inspections and drastic reduction of centrifuges etc. as I've heard about this deal, I think if they agree to all of that then I don't see the harm in allowing them some air defense. It's not like Russia is selling them fighter jets. The system would hamper us a bit should we need to go in and bomb Iran but in the grand scheme of things it would be little more than a speed bump.