There are many reasons why the US ambassador in Baghdad might have decided to give up his post, but the departure is consequential

Zalmay Khalilzad has had a penchant for calling it like is in Iraq, and at various times that has played well neither with the State department or the White House. In any case, these next few months will clearly see a reshaping of the US role in Iraq, esp. if the elections on November 7 reflect a strong Democratic showing.
WEAR ABC 3: "Iraq ambassador likely to leave, official says
November 06, 2006 12:42 EST
WASHINGTON (AP) -- Looks like the White House will be looking for a new ambassador to Iraq. A senior Bush administration official says the current ambassador will be quitting in the months ahead.

Zalmay Khalilzad is expected to take a job either in the academic world or in the private sector. Exactly when he'll leave isn't clear, but the official says he will likely stay through the spring.

Khalilzad has been in Baghdad since April of last year. He's a Sunni Muslim who at first was criticized by Shiites for favoring Sunnis. But now, the Sunnis are complaining the U-S isn't doing enough to protect them from Shiite death squads."