Having worked fairly extensively on refugee issues in the Middle East, let me let out a couple of notes of warning.

1) US involvement in large-scale repatriation and resettlement programs would immediately and inevitably invite accusations of American complicity in ethnic cleansing and demographic engineering. This is especially true given that the majority of refugees (and IDPs) have fled mixed areas, and would likely be resettled in "own group" areas. This would need to be addressed with enormous care.

2) There would be a substantial social stigma associated with containerized housing. IDPs might accept it as a location of first refuge, but I think you would run into problems resettling or repatriating currently-housed refugees and IDPs (many of them from middle class backgrounds) into it. Moreover, it is a method that would not be able to make use of local labour and housing construction skills. Cinder/breeze block construction is familiar, cheaper, easier to maintain long-term, uses local labour, works well in hot climates, and is more widely accepted. (We've looked at both prefab and wooded temporary refugee rehousing in north Lebanon in connection with the Nahr al-Barid crisis, and neither proved suitable or acceptable to refugees themselves.)

3) Don't assume the local population would welcome this, especially in a context of existing high local unemployment. Refugee quick impact projects (QIPs) can provide some immediate employment, but long term you need a viable local economy.

4) These things are always far, far more expensive, and complicated, than first estimates.

Overall, I'm not sure that the US or the Iraqi government, would gain any popularity from this. (For a domestic parallel, think how popular FEMA became after Katrina!)

That having been said, you're absolutely right to flag the magnitude of the refugee/IDP crisis, and the need to devote creative thinking (and greater resources) to addressing it, both for political and humanitarian purposes. It often gets forgotten.

Great links too!