The Obama administration's new nuclear arms reduction agreement with Russia has been beset by a chorus of conservative claims that it will "constrain" U.S. efforts to develop missile defenses, in the words of Charles Krauthammer, among many others. (By the way, this conservative love of missile defense is based on an exaggerated sense of what such a system could realistlically achieve, not to mention an unjustified faith in the possibility of a technical "fix" to the nuclear problem).
Over the longer run, when the U.S. and Russia seek deeper cuts in their respective arsenals, missile defense could become an issue, but it is not an issue with regard to the new Strategic Arms Reduction Treaty (New START) now up for Senate ratification. As Obama National Security Advisor James L. Jones explained in a recent letter to the Wall Street Journal, the treaty says nothing that would impose any real limits on current or future missile defense plans.
So, is there an issue with missile defense development and New START? No.
That being said, some observers may be confused by recent Russian rhetoric on the topic, which essentially says that if U.S. missile defenses are developed in ways that threaten Russian security, Moscow might withdraw from the treaty. Basically what this means is that if the United States radically increases the size and scope of its missile defense system in a way that might give it a first strike capability against Russia, Moscow would consider withdrawing from the New START agreement.
What would a first strike capability entail? Basically if Russia thought that the United States could strike its nuclear forces first and then block whatever Moscow had left with a missile defense shield, it would feel vulnerable to a potential U.S. attack. All of this is highly theoretical at this point (or at any point, given the technical limits of U.S. missile defense systems), but military planners deal in worst case scenarios. So if Russian leaders thought that there was even a remote chance that U.S. missile defenses would give Washington an edge over Moscow, they might want to build new and different kinds of missiles to head off that possibility.
All of which is to say that if the U.S. and Russia seek deep cuts beyond the numbers in the New START agreement, there will have to be some sort of understanding reached on the extent and purpose of U.S. missile defense systems. Suggestions of how to go about this have ranged from limiting the number of U.S. interceptors that can hit Russian missiles to developing a joint U.S.-Russian missile defense system aimed primarily at Iranian missiles (of which the long-range variety have yet to be built). Lisbeth Gronlund does a good job of explaining the challenges involved in the Union of Concerned Scientists new blog, "all things nuclear."
In short, any effort to make missile defense an issue in the upcoming ratification debate over New START would misrepresent the reality of the agreement, and should be cast aside accordingly. But if there is a new round of much deeper cuts in U.S. and Russian arsenals, missile defenses will have to be addressed.