Volokh has the numbers:
Grim Milestone Reached: US Deaths in Iraq Surpass Worst Month in Vietnam.--
As the War in Iraq approaches its third anniversary on March 20, 2006, U.S. military deaths in Iraq have just passed another grim milestone. The worst month of U.S. military deaths in Vietnam was May 1968: 2,316 lives. The second worst month in Vietnam was February 1968: 2,293 lives.
According to the website ICasualties.org, which tracks U.S. military deaths in Iraq, the total U.S. military deaths in Iraq since March 20, 2003 is 2,317 lives, one more than the worst month in Vietnam. In this count of 2,317 deaths, ICasualties.org includes 5 reported U.S. deaths pending Department of Defense confirmation.