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.