I have been trying to follow what's going on at the crippled Japanese nuclear plant, but it's very difficult because the press is being so hysterical. I'm not sure I'll ever be able to look at the Drudge Report again (yes, I still have the habit of checking it several times a day--I mean, it is a useful headline summary under normal circumstances). Here, via someone on Facebook, is sci-fi/sci writer Jerry Pournelle with a common-sense worst-case summary.
This thing is bad, but it is not a planet-threatening event. It's not even a health-threatening event right now to anyone who isn't pretty near the reactor(s), and even if the worst does happen, it will only be a serious health threat to people near enough to get a big dose of radiation before it gets diffused and distributed enough to make it pretty harmless.
There's something a little crazed about the way the press is focused on this when Japan's problems are so much greater. They talk as if "radiation" were a single thing, like a rattlesnake--you either get bitten or you don't, and if you do get bitten, you die.