It is also highly unlikely that there will be an American pope.
I think a lot of the vatican's concern with this is that American Catholicism has been one of the most highly recognized voice for change in the catholic church.
As stated on numerous news broadcasts of the Pope's memorial, the Pope's job was to maintain what God had taught. It never mattered who agreed with it. It's not the Pope's job to please people, but instead to tell them how it is, or how it's suppose to be. Religion is not a democracy.
I'm sure that one of the concerns of allowing an American Pope would be the possibility of the Pope changing or swaying in the direction of popularity, rather than that of God's teachings.
This is one of the things that made Pope John Paul II one of the most respected figures of our time. He never strayed from what was right just to please the ever changing society of people who wanted him to give his blessing for abortion, or his blessing to change how Catholicism thought about birth control, etc. He instead followed what God had written, rather than what society requested.
Interestingly enough, with that in mind, it was reported very briefly on MSNBC this morning about George W. Bush's relationship with the Pope, and they had stated that the Pope indeed gave his blessing to Bush in regards to the war in Iraq and the war on terrorism, calling it a "Mission for peace and freedom." but warned Bush that vengence could not be a driving emotion behind his decision.
I found that to be a very interesting little factoid, knowing that the Pope never made a decision based on popular belief, rather what God has written.