Let's hope they avoid meltdown. With Iodine and Caesium (fission decay products) detected in the surroundings there has clearly been some breach of the containment vessel. Whether this was controlled venting from earlier or an aftermath of the explosion this morning remains to be seen at this time.
Regardless of the eventual outcome, this disaster looks set to seriously damage the case for new nuclear power stations. Supporters (including myself) have made the point that since Chernobyl, no-one has died due to reactor malfunction anywhere in the world. In addition, coal-fired stations emit far more (natural) radioactivity in operation than nuclear stations.
The rebuttal has always been that a nuclear reactor contains many tons of extraordinarily dangerous material just waiting to get out. We now know from Fukushima that reactors are not fail-safe: in the event of a sustained power-outage the reactor is not stable.
It would not be surprising if the price for any further reactor build-out (if that's even politically possible) is that designs must in future be inherently fail-safe. However hard that might have been in the past, and Fukushima 1 is 40 years old, it must be possible now.
The Wikipedia article is being updated in near realtime. Here's a pictorial guide to the workings of a pressurised water reactor like that at Fukushima.