I think Alexander Golts has got this right:
What motivated Medvedev to surprise the world with his Iskander card? Most probably, he was determined to complicate and aggravate Russia’s already tense relationship with the United States as much as possible in the days following Obama’s election. After all, the siloviki were hoping that John McCain and a neocon Cabinet would come to power because they would guarantee four more years of heated anti-Russia rhetoric, expansionism and unilateralism. This, of course, would be grist for the siloviki mill, which needs the image of an aggressive U.S. enemy to carry out its ambitions political, defense and foreign policy programs.
A more dovish President Obama makes this task more difficult. By convincing Medvedev to immediately go for the jugular the day Obama was elected president, the siloviki were probably hoping that by provoking the new U.S. president with the threat of Iskander missiles in Kaliningrad, this would force Obama to take a more hawkish stance toward Russia. In this way, Russia could help turn the clock back to the good old neocon days under the administration of George W. Bush.This seems to be the best explanation of why Medvedev pulled the Iskander missile out of his holster on Wednesday. But Medvedev, of course, was not the main hero in this screenplay. It is clear that this plan was designed by a person who is much more powerful — someone who has clear plans to take Medvedev’s spot, perhaps earlier than we had expected.