The U.S. military should leave when one of the following occurrs:

1) The Iraqi government asks them to. This hasn't happened yet. Perhaps after the new government that will be elected in December takes office it will, or perhaps it won't.
2) The Iraqi government is fully prepared to be self sustaining and protect its people (as much as possible) in face of whatever insurgency exists in the country, and in face of external threats such as from neighboring countries (Iran, Syria). This won't be until the government that will be elected in December comes into office (sometime in 2006).
3) Iraq shows that it is unwilling to take the steps to sustain itself and defend its people. This isn't happening, but if it did, the U.S. would have to reevaluate.

Note that none of this depends on defeating the insurgency, capturing Al-Zarquawi, etc. When the nation is capable of standing on its own again, then our obligation to remain is over.

-- Jeff

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"Government is not the solution to our problem, government is the problem." --Ronald Reagan