The good news for Snowden is that, if he does secure asylum status somewhere, that would protect him from being extradited. The bad news is that it's possible that he may not actually meet benchmark legal requirements for asylum, as defined under international law, and may in fact be disqualified by the same leaks that sent him fleeing in the first place. Any country where Snowden seeks asylum would have to determine for itself whether or not he qualifies. But it's worth noting that he could face some significant challenges in arguing that he qualifies for political asylum.
Asylum is just not as simple as landing in a foreign country and asking for special status and permanent residency just because you did something that's a crime at home but legal elsewhere. If it were, Canada would be overflowing with American drug users looking to escape possession charges. Specific, established international law defines who qualifies for asylum and who does not. That law is interpreted differently by every country, but there are some general standards that don't fit easily to Snowden's case.
The right to request asylum, and the responsibility of a government to grant it, were codified in something called the United Nations Convention Relating to the Status of Refugees, which most of the world signed in 1951. (In 1967, the U.S. signed a U.N. protocol expanding the original convention.) The rights of asylum-seekers and refugees derive from that convention.
For someone to qualify for asylum under international law, he or she has to meet one of the requirements for an asylum-seeker. That means, according to Georgetown Law fellow Laila Hlass, "a well-founded fear of persecution in their home country due to race, religion, nationality, membership in a particular social group or political opinion." While Snowden may have been motivated by his opinion, any U.S. legal case against him would likely hinge more on the fact of his leaks than whatever beliefs drove them. (Put another way, if someone sabotaged a U.S. drone base and was indicted for destruction of property, he would have a tough time claiming he's being persecuted specifically for his opposition to drones.) And Snowden would have a tough time claiming he's being persecuted for his race or religion.
Snowden's best case could be to claim membership in a persecuted social group, perhaps by arguing that the U.S. persecutes whisteblowers. Usually the "social group" standard for asylum might apply to groups like the Falun Gong, a Chinese spiritual organization that suffers routine oppression, imprisonment and torture. But how do you tell the difference between a persecuted group like the Falun Gong and, say, the Hell's Angels, whose members might be occasionally imprisoned not because the U.S. persecutes motorcycle-riders but because they commit actual crimes? In other words, how does Snowden prove that the U.S. might want to arrest him not because he broke the law, which is not persecution at all but just the routine functioning of the American legal system, but because he belongs to a persecuted social group?
Hlass cited, as an example of how U.S. immigration courts have looked at asylum-seekers on the basis of membership in a persecuted social group, the groundbreaking 1990 case of Fidel Armando Toboso-Alfonso. A Cuban, he sought asylum in the U.S. on the grounds that Cuban police and officials were persecuting him for his sexual orientation. The issue is that Toboso-Alfonso actually did break a Cuban law against homosexuality, and was being prosecuted according to that law. But a U.S. immigration court, and later the attorney general, ruled that the U.S. should consider homosexuals to be a protected class, thus qualifying Cuba's laws as persecution.
One lesson from the Toboso-Alfonso is that the U.S. could only qualify him for asylum by deciding to consider homosexuals a protected social class. For Snowden to qualify under a similar interpretation of international asylum law, he would have to find a country that could fit him into a protected class. It's not as simple as a country raising its hand and offering asylum; they need a legal basis, and in Snowden's case, that probably means qualifying him as part of a specific social group that's persecuted within the U.S. Snowden's supporters might believe that the U.S. is unfair to leakers, but that doesn't necessarily make leakers a protected social group, nor does it necessarily rise to the level of persecution.
In any case, Snowden could potentially be disqualified from seeking asylum by a rule in the 1951 UN convention that establishes the international legal standards for asylum. According to article one, provision F, the rights of an asylum-seeker do not apply to people who have committed war crimes or has "committed a serious non-political crime." In other words, if a Russian dissident is indicted for insulting President Vladimir Putin, that's a political crime and it doesn't bar him from seeking asylum. But if that same dissident sets off a bomb outside the Kremlin, he can't then escape the terrorism charges by seeking political asylum abroad.
Snowden's alleged crime was releasing classified information, and the U.S. most likely considers it a serious one. The question is whether or not it counts as a non-political crime. If it did, he would be barred from receiving asylum. So how do you tell? A recent ruling from the U.S. Board of Immigration Appeals argued that a crime crosses the line from political to non-political when, as one legal group paraphrased, "the seriousness of the criminal acts outweighs the political aspect of the conduct." In other words, if you knock out the windows of a political party's office because you don't like that party's policies, that could count as a non-political crime and bar you from receiving asylum because, even though your crime was politically motivated, it was still more criminal than political. The immigration court was looking specifically at a political activist from Côte d'Ivoire who had burned some cars and buses to protest his country's government (no one was hurt). His crimes were political but the court ruled that they barred him from receiving asylum because the "violence and destructiveness of the crimes, and their impact on civilians, were disproportionate to acknowledged political objectives."
The question an immigration court in Iceland or Hong Kong would have to consider is whether Snowden's leak was more criminal or more political. Leaking classified information is not the same as burning cars, but they're both illegal. The court would also have to determine whether the U.S. was prosecuting Snowden for breaking the law or persecuting him for being a member of a targeted social group. Is he more like a Russian dissident who blew up a government office or a Cuban homosexual who refused to live in the closet?
Still, different countries can and do interpret the international law around political asylum differently. It's certainly plausible that a country like Iceland might have determined that foreign whistleblowers count as a protected social group who are wrongly persecuted abroad, although this would have to be a legal standard above and beyond, for example, government ministers opining that the Obama administration should ease up on leakers.
But even if Snowden's legal case is difficult, countries are certainly known to bend the law a bit to serve political ends; it's not unforeseeable that a foreign government could override some of the details of asylum law to grant Snowden refuge if it wanted to. Ecuador, for example, has sheltered WikiLeaks chief Julian Assange in its London embassy for several months now, apparently for political purposes.