From the New York Times, and elsewhere:
An explosion at a nuclear power plant in northern Japan on Saturday blew the roof off one building, brought down walls and caused a radiation leak of unspecified proportions, Japanese officials said, after Friday’s huge earthquake caused critical failures in the plant’s cooling system....
The Associated Press reported that the damaged building housed a nuclear reactor, though that report was not immediately verified by nuclear officials. The cause of the explosion was unclear, with some experts speculating that it may have resulted from a hydrogen build-up.
It is not clear at this time that an actual meltdown has occurred, but this is a very worrisome development—potentially indicating a partial meltdown. The Japanese government is indicating that the explosion is a likely indicator of a partial melting of the plant's fuel rods.
I'd counsel against panic at this time. If you are in the nearby region, including in the Pacific (North) West or Oceana—right now you should consider taking a multivitamin—better yet, a potassium iodide supplement. Radioactive cesium and iodine, if released in significant quantities into the environment, can replace non-radioactive potassium and iodine in the body. By taking a vitamin now, and over the near future until the situation settles, you can flood your body with non-radioactive variants and reduce risk of cancers and other organ damage. Likewise, radioactive strontium can replace non-radioactive calcium. Tums can flood your body with a little calcium to keep the strontium out. Do not over do it. Take a normal dose of any vitamins or calcium—it's sufficient to protect you.
I will emphasize again: based on the information available now, the risks seem quite low. If you are getting anxious, and want to do something productive about (what undoubtedly will be hyped up), taking some vitamins is something of use, and low risk.
I want to emphasize that there is no reason to think at this time that this is currently, or will become, a disaster on the scale of Chernobyl—a graphite core reactor without a containment building, quite different from the imperiled Japanese reactors. While a radiation leak has occurred, the volume (on a global scale) of material seems small in comparison to that disaster.
My heart goes out to the workers at these plants, struggling to contain and control the situation. They are endangering their lives and health to save us all. I'm deeply moved by their sacrifice.