The way that nationalism enters is that people feel that criticism of the verdict (especially coming from outsiders, as it mainly does in this case) is an attack on the nation's institutions (specifically its justice system), and thus on the nation itself.
In other words, if Knox and Sollecito were innocent, that would be a soldier for foreigners to use against Italy, and thus must be defeated at all costs (Sollecito's nationality notwithstanding).
Well, Italians tend to be quite un-nationalistic in that sense, but yeah, I'm under the impression that some of them would be pissed off by foreigners criticizing Italian institutions even though they would agree with the same criticism coming from a fellow Italian. (As someone-I-can't-remember-and-Google-is-failing-me said, insults hurt more when they're truthful.)
EDIT: What paper-machine said seems very relevant. Italians tend to be very pissed off when they perceive Italian institution to succumb to foreign pressure.
Today an Italian court has declared that Amanda Knox is, once again, guilty. She did not attend that trial (is not required to in Italy), so her final verdict will be either by appeal to the Supreme Court of Italy or the US extradition court. Extradition requests might be impeded due to the fact US does not have double jeopardy.
Previously on LessWrong, in The Amanda Knox Test: How an Hour on the Internet Beats a Year in the Courtroom there was some complaint that it actually took more than an hour on the internet to thoroughly research the case. Of course, the courts have been at this since 2007...
Her co-defendant, Raffaele Sollecito, who did show up at the trial, got sentenced to 25 years, but I don't know for sure where he is now because apparently he's totally unimportant and who cares (the media's opinion, not mine). I'm fairly sure he's in Italy though. So far it seems the plan is to revoke his passport but not arrest him.
Anyone want to take their hand at making predictions?