"Federal authorities said on Monday that they had uncovered no evidence that any of the surveillance activities described in the documents was currently under way."
This was, of course, widely predicted after the New Republic story that an important al-Qa'ida member would be found in Pakistan shortly after the Democratic Convention. Personally, I think the last thing to fear is another skyscraper bombing - they don't repeat themselves, and there are plenty of really serious industrial targets where nothing much has been done about security.
The US alerts remind me increasingly of the way investment analysts in the boom began issuing all kinds of new ratings instead of plain old buy or sell - "accumulate", "strong buy", "market perform". Strangely, they were all on the upside. There wasn't a "strong sell" or "dump this dog right now and stockpile canned food" rating. More recently, several of the investment banks have set up rules stipulating that the number of sells must equal the number of buys. The terrorism alert levels have this problem. It only goes up. A sensible version would be balanced around normality (VERY LOW, LOW, NORMAL, ELEVATED, HIGH) - but that would make it far less sexy.