The nuclear disaster in Japan has increased from a 4 to 5 on a scale of 1 to 7.
These are categories on the International Nuclear and Radiological Events Scale (INES), which runs from zero (nothing happened, essentially) to seven, a "major accident".
So far, Chernobyl is the only seven-rated incident in nuclear history.
Level five is defined as an "accident with wider consequences".
[Source, BBC News Science & Environment, Japan raises nuclear alert level]
Three Mile Island was also a level 5 incident. Now the radiation has reached the United States.
[1:39 p.m. ET Friday, 2:39 a.m. Saturday in Tokyo] Monitors in Sacramento, California, have detected a small amount of radioactive material from the earthquake-struck nuclear power plant in Japan, an official with the Comprehensive Test Ban Treaty Organization said. The exact amounts were not available, but were far less than what would be considered harmful to human health, the official said.
[Source, CNN, Japan quake live blog: Small amount of radiation reaches California, official says]
Before you put on your tinfoil hats (and remember, mine is shinier than yours), and cover your testicles in aluminum foil, re-read the sentence that the radiation is "far less than what would be considered harmful to human health."
Update: Another doctor cautions against over-reaction:
"I think the biggest health impact is the psychological impact," said Dr. Jonathan E. Fielding, the director of the Los Angeles County Department of Public Health. "Anytime people hear ‘radiation’ it evokes a level of fear that is disproportionate to the threat."
[Source, The New York Times, For Radiation, the Alarm Bells Are Boxes]