The AP reports that Homeland Security Secretary Michael Chertoff "raised the threat level for flights from Britain to 'red,' designating a severe risk of terrorist attacks. All other flights, including all domestic flights in the United States, were put under an 'orange' alert—one step below the highest level." This in response to the news that British officials have nixed a terrorist plot to blow up several planes flying to the U.S.
So just in case you've forgotten what the government's color codes mean (and for all the good it will do you) I supply the link to the Department of Homeland Security's Citizen Guidance on the Homeland Security Advisory System.
As someone who is about to fly to Europe in the next two weeks, I will be following this story with particular interest.