There are plenty of reasons to be skeptical about the Wall Street Journal article that describes "U.S. worries" over alleged movement of Russian tactical nuclear warheads. The timing is, of course, one - the story "conveniently" surfaced at the time of the debate over the New START. But there are others as well.
First, the story seems to conflate missiles and warheads, so it is never quite clear what it is that was moved. The headline mentions missiles, so it's seems that Russia redeployed some Iskander short-range missiles from one place to another. As any reasonably advanced missile, Iskander is probably capable of carrying a nuclear warhead, but it has never been deployed with one. And there are good reasons to believe that no nuclear warheads are assigned to these or any other short-range missiles and definitely no warheads follow the missiles in their movements.
Then, the story alleges that Russia violated a pledge that it never actually made - "to pull tactical nuclear weapons back from frontier posts." What Russia promised was to move its tactical weapons to centralized storage facilities, wherever they are located. And Russia is on record as saying it has done so - the most recent statement that I can find was made by Sergei Ivanov at the 2010 Munich Conference:
Russia has reduced by three quarters its tactical nuclear arsenals and concentrated them in central storage bases exclusively within its national territory.
In fact, there are not that many storage sites there, so even if you move warheads from one to another, the distance to NATO wouldn't change much (a good list was published in the Nuclear Notebook).
Some statements in the story are highly suspect. For example, it says that the United States believed that Russia "has expanded tactical nuclear deployments near NATO allies several times in recent years." But all the evidence that supports this statement is an April 2009 State Department cable that says that Russia made a threat to deploy missiles in Kaliningrad if the United States goes ahead with its missile defense deployment in Europe. We know, of course, that the United States changed its missile defense plans since then, and shortly afterwards Russia said that it sees no need to deploy anything in Kaliningrad. How all this amounts to an example of "expanded tactical nuclear deployments" in Kaliningrad completely escapes me.
Then, the story spends some time suggesting that the alleged movement was a response to U.S. missile defense after all - in May the United States deployed Patriot batteries in Poland. Russia certainly didn't like that deployment, but if it wanted to send a signal of disapproval it would have made it openly. It has done so in the past - the Kaliningrad threat was made in President Medvedev's annual address to the parliament in November 2008 (note, by the way, that neither Medvedev, nor the military have ever said anything about Iskander being nuclear):
[W]e will deploy the Iskander missile system in the Kaliningrad Region to be able, if necessary, to neutralise the missile defence system.
If you follow through on a threat like that, you would want people to notice, wouldn't you? So, any link between whatever missile movements might have happened in the spring and missile defense developments is tenuous at best.
Finally, the story makes a good point of noting that "the U.S. has long voiced concerns about Russia's lack of transparency when it comes to its arsenal of tactical nuclear weapons." No doubt, Russian arsenal could use some transparency, but I would note that the U.S. has disclosed no information about its tactical nuclear weapons either, so this also should be a legitimate reason for concerns.
So, what happened in the spring of 2010? My best guess is that some missiles were redeployed from one location to another, probably as part of the process of reorganization of the military. I'm fairly confident that no nuclear warheads were moved in the process. I believe we can also be certain that whatever happened was not related to the U.S. missile defense moves in any way. And, of course, one thing we know with absolute certainty - someone desperately wants the New START treaty to fail.