In particular the scene in which a player is put into an airport in Moscow has caused an upset, though for what specific reasons hasn't been clarified yet. Neither Activision nor the Russian government has released an official statement on what the situation is, but for now the game has been pulled from shelves in Russian stores. There's also rumors (though nothing official yet) that the game may face an outright ban.
According to some sources, a version of the game stripped of the Moscow encounter is going to be made available, replacing the already deployed Xbox 360 and PS3 copies of the game. For the PC version, whether bought from a store or through Steam, will be patched – stripping the content out without having to replace the game altogether.
Though the latter portion may not make players happy, it apparently is sating the Russian government for now, as sales of the PC version are continuing unhindered. Much of this is based on speculation, of course, until Activision pushes out a press release.