Here is a summary of what I see going on at the Fukushima-Daini reactor complex. This is based on information from industry sources and from networking with the nuclear engineering community. Plus, I am a nuclear engineer so that helps in translating. If you have any questions that pop up from reading media accounts, put them in the comments section and I'll try and answer when I can
The short version: Things are very bad, but not catastrophic. Units 1, 2, and 3 (by unit, this means an individual power reactor at this power plant site) survived the earthquake, but the tsunami absolutely devastated the supporting infrastructure that helps keep the reactors pumping water to remove the heat from the core. All three units have had the water in their core drop below the fuel rods to some extent for various periods of time and significant fuel damage has occured. That said, they are all being flooded at the moment with seawater. Their pressure vessels are intact (meaning the core itself is sealed inside a very thick steel container) and the "primary containment building" is intact (this is a building of very thick concrete surrounding the reactor pressure vessel). Radioactive material from the damaged fuel has been released when they vented some steam to reduce the pressure in containment. This is bad, but this is no Chernobyl by any stretch. The hydrogen explosions have made fearful TV, but they have not damaged the key pieces - the pressure vessel and the containment building. We would know if they had by a huge spike in contamination by Iodine-131 and Cesium-137.
No radiation in any appreciable amount is going to reach the U.S. Ignore any and all calls to take KI tablets - there is no need. Save them for a nuclear war.
You will also read sensationalist accounts that the Japanese government is lying and engaged in a coverup. This is impossible as this entire region of the world is laced with radiation detectors not under the control of the Japanese government (to keep an eye on our North Korean friends, along with the Russians and Chinese) and we would know very quickly if huge amounts of rad material were being ejected into the atmosphere.
Also please note, you are going to read a lot about detecting radioactive material, possibly even in the U.S. How does this jive with my statement above that the U.S. should not worry? We have detectors that are so precise that we can still measure the tiny fractions of radioactive cesium still present in the soil from above ground nuclear bomb tests from over 50 years ago. Yes, I am sure we'll detect something and yes, you will probably read about a "nuclear fallout cloud" circling the earth. It will be hype and fear and not relevant to your health.
I won't go into the technical details unless there is a significant demand for it. For more technical meat, check out the ANS Nuclear Cafe for stories and links.
I don't want to minimize what is going on, but while this is very bad, the engineers in Japan have done an amazing job under horrible conditions to move things towards a stable sitauation. We are not out of the woods yet, but every hour that goes by helps.