Here's a summary of the current status as I understand it. One of the things that makes following this confusing is that a number of news sources seem to be reporting things that happened yesterday as things that just happened.
Units 1-3 are being cooled by sea water. There is evidence of damage to containment in Units 2 and 3 and probably fuel damage in all three. Unit 3 is of particular concern since it's using experimental mixed oxide fuel that has some more radioactive elements than in traditional fuel.
The biggest concern right now appears to be the spent fuel pool in Unit 4. Units 4-6 were in a refueling outage when the quake happened. When a reactor is being refueled, the old fuel is moved into a large, heavily reinforced pool of water that is cooled by circulating water. The fuel cools slowly and is eventually moved to long-term storage. Without cooling, enough water boiled away in the spent fuel pool to expose the fuel and melt some cladding. There have been at least two fires in that building and possibly a hydrogen explosion.
They were trying to see if water could be dumped on the spent fuel pool from helicopters, but they determined that the existing holes in the roof of the structure, caused by either the fire or the hydrogen explosion, were not large enough or in the right place. They seem to have decided the radiation levels were too high for workers to try make a larger opening and possibly too high for the helicopter pilots. Currently, they're working on spraying water using high-pressure hoses from the defense force. They're also running a temporary line for off-site power.
Several reports have said that the "50 workers stayed behind, a crew no larger than would be stationed at the plant on a quiet spring day." On a normal day, there would be 800+ workers. The rest of the workers were evacuated. I think they're referring only to reactor operators. I'd guess that the 50 who remained are a mix of operators, the people who run the units from the control rooms, and non-licensed operators, who do a number of technical jobs in the plant. Knowing reactor operators, I'd guess that many who aren't at the plant are asking to be sent back. It's hard to convey how dedicated most reactor operators are.
There was also a report that all employees "fled" the plant, leaving the recovery effort. There was a spike in radiation and those 50 moved further away for about an hour until it went down.