Iranians/Persians are NOT Arabs. They have a distinct history, language and ethnicity. They are no more Arab than Jews and are disliked by the Arabs just as much. Some Arabs consider the Shias among them to have "hearts that beat Persian". I could go on and on but you get the picture. Don't EVER call a Persian an Arab because they consider it an insult. You may as well call a Jew an Arab. Read up on the history of the region and see if you don't get addicted to it.
I agree with much of this and bolded the part of your post that is the biggest misconception people have. If Iraqi Shias are allowed to thrive and take their natural place, to an extent they will actually become rivals to Iran as they tussle over Arab vs. Persian Shia Islam and other things. People also forget (or maybe just don't know) that Iraqi Shias (not all, of course) fought for Saddam Hussein against Iran in that brutal war. There is no doubt the Iraqi Shias seem themselves as Iraqis. At the same time, Iran is supporting them big time right now in their time of crisis and they appreciate it, but that doesn't mean they want to merge into Iran. Soft-partition keeps Baghdad as the capital but gives more autonomy to the regions. 2nd request: If not soft-partition, what course do you propose for the U.S. in Iraq?
Isn't the historical position in Iraq where one ethnicity dominates the other? Now that Iraq is 'liberated', could it be that the Shias think it's their right for their turn to lead? I think the Al Qaeda/insurgency movement will severely fracture Iraqi identity as more holy shrines are decimated in Al Qaeda's calculated move for them tear each other apart. For them to unite against the outsiders, they have to be willing to set apart their politics, compromise on power sharing, and let all of their disparate leaders save face while maintaining appropriate authority. It doesn't look good for that right now.
If you mean federalism, then that's pretty much what we're trying to do. I don't think there are any good options, just a bunch of bad ones. However, the 'least' bad option is to keep the country intact and continue on the course we're on now. However, I would seriously consider dismantling the current weak government in place and instead propping up a coalition of 'strongmen' (Shi'ites, Sunnis, and Kurds; preferably from the military ranks) that can establish security in the middle/southern part of the country and effectively enable us to take a 'backseat' or supporting role in the country. If democracy in Iraq is a big deal for us, then have an agreement in place that once security is no longer an issue, the government would slowly transition to democracy. However, democracy can't and shouldn't be a priority when its very existence is impeding progress in the country.
Has anyone considered that it might not be possible for us to solve Iraq's problems? Partition or not shouldn't it be left to the Iraqis to try to figure that out?