Kevin Drum tries to throw some water on the "Middle East in Flames" theory holding that American withdrawal from Iraq will lead not only to a short-term intensification of fighting in Iraq, but also to some kind of broader regional conflagration. Ivo Daalder and James Lindsay, as usual sensible but several clicks to my right, also make this point briefly in Democracy: "Talk that Iraq’s troubles will trigger a regional war is overblown; none of the half-dozen civil wars the Middle East has witnessed over the past half-century led to a regional conflagration."
Also worth mentioning in this context is the basic point that the Iranian and Syrian militaries just aren't able to conduct meaningful offensive military operations. The Saudi, Kuwait, and Jordanian militaries are even worse. The IDF has plenty of Arabs to fight closer to home. What you're looking at, realistically, is that our allies in Kurdistan might provide safe harbor to PKK guerillas, thus prompting our allies in Turkey to mount some cross-border military strikes against the PKK or possibly retaliatory ones against other Kurdish targets. This is a real problem, but it's obviously not a problem that's mitigated by having the US Army try to act as the Baghdad Police Department or sending US Marines to wander around the desert hunting a possibly mythical terrorist organization.
The real issue is that between the gloom-and-doom right and the modern-day decent left both emphasizing how departure will lead to bloodshed in Iraq, we've had very little recognition of the fact that how much bloodshed we're talking about is very much an open question and that we need to be thinking about how to minimize it. It's very implausible that you'd have all these countries invading Iraq. It is, however, totally plausible that Iran and Saudi Arabia, possibly with Turkey, Israel, and God-knows who else getting into the mix, might do exactly what the United States (and to a lesser extent, Iran) is already doing right now and giving the combatants weapons and money. Bigger inflows of money and weapons means a larger, deadlier civil war and we should try to stop that through diplomacy, contact groups, etc.
How effective that could be, I really couldn't say, but part of the package would have to be that we stop arming and funding the different factions. Does anyone think that the Iraq Air Force we're building is going to be anything other than a lethal participant in the post-withdrawal war? Intensified civil conflict is a real worry, but our mission in Iraq right now isn't helping that problem, it's making it worse.