It has been appropriate to withdraw for a long time now. Just in case there was any confusion left, however, a decisive event towards moving in that direction has occurred. One knows that it is time to go when the Shia leader al Sistani has declared that attacks on US Troops are acceptable.
Sistani is a moderate but extremely influential Iraqi Shia leader; easily the most powerful in Iraq. In the first few years of the invasion, he issued non-violence religious orders regarding US troops. He also kept Iraqi civil war at bay, by prohibiting his followers from retaliating against Sunni provocations. Finally, he urged his followers to vote in the US sponsored elections, which were thus a success only because of Sistani.
Hopefully the Shia government will ask US forces to leave soon. If they don't, we still should anyways. We have won the hearts and minds of nobody; the Iraqis don't want us there, and they don't appreciate the sacrifices our soldiers have made. Al Qaeda isn't going to take over the country; neither is Iran for that matter. Its long past time we let Iraqis sort things out for themselves.