The reactor on the northern coast that was in a warning state had reached 2.5 times above the maximum rated pressure earlier this evening... If they can't start pumping coolant soon, venting the "radioactive steam", isn't going to be enough...
They began "venting" as a last resort, there's also another 7 reactors/stations in the same bay...
The New York Times has an interesting (and sad) graphical comparison of satellite photos of various points in Japan before and after the quake, where the two are exactly overlaid and you can slide the before vs. after line around.