Commentators seem to say that it was a real acquittal. (Compare to Michael Morton and Ryan Ferguson cases in the U.S. on the TV blog under “wrongful convictions” label, also look at the Innocence Project and the film “The Exonerated”.)
Update: Sat, Feb. 1
On CNN, Richard Herman and Avery Freeman disagreed on whether the State Department would feel forced to extradite Knox if the conviction is upheld in Italy. Herman didn't seem to give much credence to the double jeopardy argument.