It's Tuesday afternoon here and I'm watching on TV and internet the situation in Japan, and reading between the lines...those reactors are nowhere close to being under control. I'm not sure the ISU can get enough clarity within the next few days to make any decision that keeps the W.C. in Tokyo. Theoretically, if a suitable venue was available, it might be possible to hold Worlds at Osaka or points west within Japan which might keep existing sponsors/TV happy, but I suspect there is now a lot of psychological resistance among the non-Japanese participants (incl national federation officials, judges, some skaters etc.) to going anywhere in Japan. In short--I don't see the ISU holding this event in Japan this year.
I think the US is likely a non-starter for emergency hosting due to arduous visa requirements...and unfortunately the US State Dept consular employees out in the field are unlikely to make special expedited dispensation for affected athletes/coaches/officials. They theoretically could, but in practice, they won't. Based on what other posters have said, Canada is not likely to want to be an emergency host. I think Korea could physically pull off an emergency hosting, and I'm sure China (Beijing) could also, as long as the will and the support is there from the government and from the top of the relevant sports authorities. I think there would be plenty of fans in the seats in either country--though that probably ought to be the least of the ISU's worries right now. I don't see security being too much of an issue in either of these countries, and knowing China, they could have several hundred volunteers ready to rock and roll in a week. I don't know about hotel space in the Korean cities, but Beijing has a ton of available rooms within reasonable distance of the likely venue. I'm not aware of much of anything going on at that (18,000 seat) venue until May 1. South Korea has a large number of visa-free entrants allowed and deals with visas where needed decently. China requires visas but can process them very fast and mostly painlessly. I don't know enough about the European options to have an opinion.
I guess I'm leaning towards the school of thought that says the ISU should try to hold the WCs somewhere if humanly possible, even if it is a stripped-down version focused only on the most critical elements necessary for competition, and jettison the frills. I'm also taking some cues from quotes by the Japanese contingent of skaters, who (except for Murakami) seem to be OK with the event going forward.