I know this discussion is a month old, but I may be able to clear it up. From the U.S. Embassy, May 10:
"The U.S. Mission in Tokyo has received information, through the Internet,
about a possible bomb attack against the American Embassy during the week of
May 10.
We are unable at this time to determine the credibility of the threat, but
we feel it is our responsibility to take all such information seriously and
to share this with the American Community. The appropriate Japanese
authorities have been notified."
But not to worry. These notices come through every few weeks and have been for a long time now. If you'd like to get frightened regularly, you can sign up to be on the emailing list and get the Embassy security warnings as they come out.