Here's an interview with a retired scientist from Sandia Labs whose specialty was the catastrophic failure of nuclear reactors:

http://blogs.knoxnews.com/munger/2011/03/ex-sandia-engineer-talks-about.html

tl;dr -- it will be months before we know whether the power company will get control of the situation. Best case scenario: what we've got now -- a failed reactor with damaged rods which will take years to clean up and dispose of. Worst case scenario: a Chernobyl style explosion which releases a plume of radiation which drifts south over the Japanese island. (If the plume drifts over the Pacific, Dr. Allen says that's a good thing because it will disperse harmlessly.)