The number of Americans killed per month is an imperfect measure because there are many things American commanders can do that affect the figure regardless of the level of Iraqi violence. The April 2008 Aerospace America, primary publication of the AIAA, America's primary professional association for aerospace workers, detailed one of them.
The article concerned problems with the US fighter aircraft fleet. The older fighters are wearing out, in part because of being used extensively in Iraq. On page 27 the article states: "Because of a concerted effort to avoid casualties in Iraq and Afghanistan, strike missions have increased markedly." In other words, U.S. casualties are down, in part, not because of less Iraqi violence, but because aircraft strikes are used in place of ground action. That said, reducing American casualties is obviously a good thing.
Unfortunately, not only does this mean 'the surge' is less successful than advertised, it also means more 'collateral damage,' a term meaning killing the wrong people, because air strikes tend to be less accurate than foot soldiers. Killing the wrong people tends to make surviving relatives and friends distinctly more anti-American, which is how you lose counter-insurgency wars (see the U.S. Field Manual).
The lesson: when someone tells you their war is going well, look closely before you believe.