I think Mike's right that it's weird people don't seem that alarmed about Najibullah Zazi, but I also think it's pretty understandable. Part of it is the crying wolf phenomenon: After so many supposedly big-deal domestic terrorism arrests turned out to be what NYU law school's Karen J. Greenberg calls "fantasy terrorism cases" (Padilla, the Liberty City Six, the Lackawanna Six, etc.), I think a lot of people have just become inured to this sort of thing, not to mention skeptical.
What's more, unlike the Bush administration, the Obama White House doesn't seem intent on making a big deal about these arrests--even when, as appears to be the case with Zazi, the arrest is indeed a big deal. So far as I can tell, the most senior member of the Obama administration to comment on the Zazi case has been Attorney General Eric Holder who issued a brief written statement assuring Americans that law enforcement officials "believe any imminent threat arising from this case has been disrupted." Contrast that with John Ashcroft's overheated televised announcement (from Moscow, no less) of Padilla's arrest back in 2002, and the differences between the two administrations couldn't be more clear.