Wired reports on a GAO Report (full version, summary) that says the

nation’s centralized watch list has grown to include 755,000 names suspected of having terrorist ties, resulting in nearly 20,000 positive matches of persons against the list in 2006, according to a new report from Congress’s investigative reporting arm. Since the list is now used in nearly all routine police stops and for domestic airline travel, Americans made up the bulk of those matches.

The article notes that "individuals who believe they are on the list erroneously or because of a confusion over their name can apply for help through the DHS Advise system, but it is not set up to allow people to contest the information that got them on the list."

There were only 400,000 names on the list a few years ago.

I’m just spitballin’ here, but it seems to me that as more and more names appear on the list, its utility continues to diminish.  But the GAO report touches on this.