Boytwnmom, this article seems to shed more light on the subject:
"The legal question would be whether Knox was acquitted, as U.S. courts would define the term, or whether the case was merely reversed and still open for further appeal, said criminal lawyer and Harvard Law Professor Alan Dershowitz.
...
Much of the complication stems from the differences between the Italian and U.S. legal systems. In the United States, if a defendant is acquitted, the case cannot be retried.
In Italy, prosecutors and lawyers for interested parties, such as Kercher's family, can file an appeal. Unlike American courts of appeal, which only consider legal errors in the courts below, Italian courts of appeal, which are comprised of both judges and jurors, can reconsider the facts of a case.
Depending on the Italian high court's reason for overturning Knox's acquittal, it is possible that the court of appeals could consider new evidence that's introduced, said Dalla Vedova. As a result, a defendant can effectively be retried in the course of one case in Italy.
Dalla Vedova said the high court's decision does not raise a double jeopardy problem because the retrial would not be a new case but rather a continuation of the same case on appeal.
Other
defendants who have been acquitted in other countries and then convicted on appeal have attempted to raise the double jeopardy principle to avoid extradition, without much success, said Mary Fan, a law professor at the University of Washington who specializes in cross-border criminal law.
While the issue is rare in the United States, several courts have rejected the double jeopardy argument in similar cases. In 2010, a federal court in California found that a man who was acquitted of murder in Mexico and later convicted after prosecutors appealed the acquittal, could not claim double jeopardy to avoid extradition to Mexico. That court cited a 1974 decision from the 2nd U.S. Circuit Court of Appeals in New York, that reached the same conclusion with respect to Canadian law, which also allows the government to appeal an acquittal"
http://uk.reuters.com/article/2013/03/27/uk-italy-knox-extradition-idUKBRE92Q01620130327