Dr. Steven Taylor from Poliblog has posted a practical definition of civil war that everyone should read. Whether or not Iraq is in a civil war, depends on who you ask. As he points out, today it is being used as a political term, from that perspective, more so than from a pure political science one. Democrats say yes, Republicans say no.
To get a better understanding of this, I highly recommend taking a look at this essay and the one that preceded it, on the same topic.
I am not sure if Dr. Taylor would agree with me or not; but I would say that if we accept his definition, it is not quite there yet. The goal of the attacks appear to be aimed at starting a civil war and not conducting one (at least from where I sit it does). The latest wave of violence is pitting Sunnis against Shiites (and vice versa) and not directly against the Iraqi government. The reasons behind this violence is more religious than political, at least at this point in time. However, I must stress that if they are not there yet, then they are as close to it as they can be, without really being at that point.
Check it out.