Honestly, I think that there are a lot of valid points here, but here are some basic facts, as I have seen them.
1. If we were to do a mass withdrawl, Iran would probably just move in and take the land in Iraq that they used to claim..
2. An already unstable region would probably erupt into all out war, to include Iran, Iraq, Syria, Lebanon, and Israel.
3. The world would condemn the US even further, as Iraqi tribal factions slaughtered each other in moves for control of the regions they wanted to claim as their own.
Now, this is all just conjecture, at this point, but there are a LOT of eyes in the Mid East, looking for any excuse to justify their own "holy wars against all those who do not believe the exact same things they do.
Also, theres a kind of damned if we do, damned if we don't situation going on in Iraq, with US forces.. Theres a TON of misconception out there, that we are somehow responsible for millions of deaths of Iraqi civilians, where its actually the fundamentalists doing 98% of the killing over there.
So, world opinion is that we are wrong for being there, but we really cant leave yet, because we actually are the only forces keeping a lot of the crazies from going all out..
As far as withdrawl, I think a very SLOW trickling out of troops and equipment is the right thing to do, as the Iraqi Police get trained and get better equipment.. They are starting to stand up to some of the extremists, now, because the moderate civilian population as a whole is getting very sick and tired of the kidnappings, suicide bombings, and other violence in the country. People aren't hiding their support for the US nearly as much anymore, because frankly, they don't want to deal with living in chaos any more.
Just my opinion....