Over 2/3rds of those who support continued US involvement in Iraq significantly underestimated US casualties... their estimate of US losses were approximately 30% of what the actual casualties are. 25% of those in favor of continued involvement in Iraq were approximately correct, and 13% in favor considerably overestimated US casualties.
Of those against continued US involvement, about 35% significantly underestimated casualties, about 33% were approximately correct, and the remaining third overestimated casualties.
Basically, about half of those who took the poll significantly underestimated casualties. The poll also showed a significant correlation between people's underestimation of US casualties and their support for the war.
I can only make guesstimates, but it seems likely based on poll results and the level of support for the war in the US that over half the country is significantly underestimating the casualty total of the conflict by over 50%. This discrepency is almost certainly caused by how the media has been reporting casualty figures.
The actual number of US casualties in Iraq as of today is approximately 220. Given the current rate of US casualties in Iraq, it is quite likely that casualties for the first year of US occupation in Iraq will be roughly equal to all US casualties in Vietnam from 1961 to the end of 1964.