In the process of looking for an estimate of the number of "insurgent" detainees in Iraq, I came across the Brookings Institute's Iraq Index. Updated three times a week, the index contains figures, charts and survey results ranging from carnage to war profiteers. It's pretty fascinating (in that car wreck sort of way). The table below, for instance, comes from a survey at the end of June 2004.
I still don't know the answer to my question, but I do know we're averaging between 2,000 and 3,000 "insurgents detained or killed" (no differentiation in status here) and there are 7,900 people in prison in Iraq.
From the Iraq Index