Senator Biden's idea for a partition of Iraq is behind the data curve. The pattern today is that Iraqis who move are often fleeing lack of critical services (e.g. electricity, security protection.) Ethnic/sectarian differences are not the deciding factor in the relocations. Sunnis are moving into Shia neighborhoods and vice versa.
I don't have the link to the report that supports this observation, which I came across this week or last at Iraq Slogger; the link got lost in the flurry of links I collected during the past week on the situation in Iraq.
However, given the length of time that Iraqi Sunni and Shia lived in relative peace with each other prior to and during Saddam Hussein's regime, the report makes sense.
The report does not speak to ethnic cleansing, which has been carried out in many places in Iraq, and particularly during 2006. It does indicate that given enough freedom to decide, many Iraqis think more in terms of economic class than along ethno-sectarian lines. In other words, they choose to live in the best neighborhood they can afford.
Speaking of Slogger, I have advised several times that the website is an invaluable tool for keeping up with fast-changing events in Iraq. Granted, plowing every day through the mountain of data at Slogger is a job. But Slogger makes the chore as easy as possible by providing well-written daily comprehensive summaries of news, and with links to the source reports, if they don't come from Slogger staff.
I also realize that not everyone can afford a subscription to Slogger, even though it costs nothing to peek at their headlines and click on the feeds. Yet one does expect congressionals who are trying to influence the Iraq government to keep up with the times. There is no reason why US congressionals can't subsribe to Iraq Slogger, and have their aides read Slogger's daily summaries. Joe Biden, take note. Often, congressionals are reacting to outdated or incomplete data.