Better safe than sorry.
I just went over to Debka -- they seem to think it's about a "dirty bomb" or chem/bio attack, so I can see how we would over-react, rather than take a chance of ignoring a warning.
Here is the particular excerpt from Debka:
"DEBKAfiles counter-terror sources reveal that US counter-terrorist authorities are focusing on tracking down two key suspects: a 32-year old Pakistani woman called Aafaya Siddiqui, doctor of neurology who attended a university in the Boston area, and Adnan G. Al Shukrijmah, an al Qaeda activist known to have spent time in Florida.
Our sources add that American and foreign counter-terror and intelligence agencies are also hunting for Midaat Mursi, known as Abu Khabab, prominent member of al Qaedas operational partner, the Egyptian Jihad Islami, who is considered the organizations top expert on radiological bombs. Mursi heads a number of terrorist rings who have been instructed to infiltrate the United States through Canada for a dirty bomb attack. They may have made the crossing already.
The price on the head of Musab Zarqawi, Osama bin Ladens top operative in Iraq, may be raised from $10m to $25m after he was identified as the terrorist who beheaded Nicholas Berg. All his nets are the subject of intensive search, some composed of Saudis and Chechens trained for chemical or biological attacks and buried somewhere in Europe for some time. Efforts are also underway to locate underground terrorist cells of al Qaedas Far Eastern ally, Jemaa al Islamiyah, which are thought to be on their way to seize a merchant freighter or supertanker for use as a massive seaborne bomb vessel.
Several sleeper cells are also known to have infiltrated the United States in ships containers in 2003 and early this year. Some of these containers were spotted at important US ports with signs of occupation by men with weapons and explosives who were never caught. They may have been decoys to distract attention from landings of large parties of armed terrorists on American shores, whom intelligence sources believe headed undetected for safe houses inland to await orders to strike.
These are others are some of the unidentified enemies that Ashcroft and Mueller will ask Americans to watch out for this summer. "
http://www.debka.com/article.php?aid=854