Dan Darling has an important piece at Threatwatch that analyzes whether the sectarian violence in the aftermath of the Samariyya bombing amounts to a genuine civil war or not. FWIW I agree that the sectarian violence after the Samariyya bombing probably did not amount to a genuine civil war, and that if anything Iraq has already been embroiled in a low-level civil war of sorts ever since the first post-invasion bombing at the shrines of Najaf that killed an ayatollah (Hakim?).
However, my opinion is that if a successful destruction of the shrines in Karbala or Najaf is carried out, then we will assuredly see a true and genuine civil war, with likely breakup of the country resulting. I've previously mentioned that in my view, partition of Iraq is tantamount to defeat of our goals (however you wish to define them).
Also, I don't really buy into the Iranian angle for Samarriya, as others have suggested. If anything I think that Iran benefits mroe from a united Iraq dominated politically by the Shi'a rather than a sectarian breakup. I don't see how a civil war really benefits Iranian aspirations; in many ways it actually hinders them, because it would create a hostile Sunni state on their doorstep.