The following is entirely from Media Matters. Read the entire article here. A synopsis:

Media outlets have erroneously likened reported voter turnout in Iraq to that of the United States. According to the Independent Electoral Commission of Iraq, 58 percent (or 8.55 million) of the 14.66 million Iraqis registered actually voted on January 30. But contrary to assertions by numerous media outlets, this figure is substantially lower than voter turnout in the U.S., which in 2004 was approximately 122 million out of the approximately 173.6 million registered voters, or 70 percent. Between 1992 and 2000, the percentage of registered voters who cast ballots in U.S. presidential elections ranged from 66 to 78 percent.

In making the erroneous comparison, the media presumably relied on the more typically reported 2004 presidential election figure of 60.7 percent turnout in the 2004 presidential election. But that percentage is not of registered voters; it is of a larger pool of eligible voters, numbering approximately 201 million.