Iraq has been a very mucky war and will remain a diplomatic mine field for a very long time to come.
Obama is coming up for re--election so he has to show that he is full filling his promises, however, I doubt we will see a complete withdrawal for quite sometime, what I can see on the other hand is that 'an event' may occur which, for the sake of national security will require a permanent base in the country, the heat being turned up on Iran may well trigger that event...
Having witnessed from the inside the collapse of the Soviet empire I remember panic and disorder in the government that started few years before it happened. My prediction is that the "American era" still continues for a while.
Certainly the withdrawal of troops from Iraq is more symbolic rather than anything. The US still have a major embassy compound, have US contractors and consultants involved in Iraq and also will guide the country and its rulers through foreign aid. So its not a total withdrawal, it serves as a nice talking point when Obama runs his re-election. The US will maintain its influence in the Middle East. However with regard to the waning US influence around the world, I would not opine that is a weakening of the US than it is more an awakening of the BRIC countries gradually over the years and the West not developing at the same speed as emerging markets. Eventually, I think the US will become a regional power with a strong but not dominating presence on the international stage. But its "fall" will never be catastrophic or spectacular like the fall of the Soviet Union. It has been written before that the US will fall from its Hedgemonic status, multiple times during the cold war, or during the 80's with the rise of the tiger economies and each prognosis has been wrong. I guess one could argue that especially the economic factors would make a stronger argument now but the anticipation from many to witness the US fall will always be there. Haters.