Obama recently sent a letter to Moscow. Presidents do this quite often. As far as I know, we even have a red phone to Moscow in the Oval Office today.
But this letter is a special one. And it was supposed to be a very quiet one. Somehow, through journalistic voodoo and heka magic, the New York Times happened across the contents. It looks, for the moment, that Mr. Obama was quietly implying that the US would halt and undo its missile defense placements in Europe in exchange for Russia making sure that Iran would not be able to build ballistic missiles.
The implications are interesting. First, about the leak--it is unclear whether the letter was supposed to be leaked or not. Intentional leaks are common, in order to let the state throw its hands up and say, "we didn't mean to publicize this," and so further questions are not pressured. This may be an intentional leak--it may be an attempt to show Eastern Europe, Russia, et al, that the missile shield is not intended as an anti-Russian initiative (even though it is). This potentially gives Obama some political traction for continuing the project, should it seem necessary.
But it also makes the shield a bargaining chip with Russia. Obama says, "we know you don't like it, but hey, it's for Iran, so if you can just make that problem go away, we won't need it anymore, and we'll stop spending money on it." It means that, if the Russians choose to (and indeed, are able to) deal with the Iranians, then Obama is willing to reprioritize on defense. The likelihood that the Russians are actually going to engage in ballistic missile brinksmanship bargaining is low... but it is not so low for the Iranians. Furthermore, the EU is actually collectively strong enough to stand up to the Russians if they are willing to get their hands dirty, where Israel is less able to do so (even if much more willing).
So Obama knows that today, one needs to sidle up to the Russians, and pressure the Iranians. Obviously, this is tough to do. Medvedev, when questioned about the leak, mostly shrugged at the situation and said that "nothing concrete" was in the letter. He knows that he is in a relatively good relative bargaining position, and is not ready to yet show his hand.
But ultimately, the missile shield in Eastern Europe is something that the Russians are willing to do a lot to get rid of. In a somewhat ironic way, Mr. Obama can thank Mr. Bush for leaving the incomplete missile shield behind so that he, in the nicest way possible, can use it as a lever to pry Moscow.