Golly -- This would take a year to explain, but here's the Readers Digest version (There are dozens of court cases and laws governing this practice, but I can't take the time right now to find them all again):

1. Airports are considered to be "functional equivalent of borders". The government can do everything at an airport that they do at a physical international border, such as the numerous border crossings between the U.S. and Canada or Mexico. This application of law has never been questioned, as far as I know.

2. The government has used this "functional equivalent" definition to justify the internal border checkpoints that exist in California, Arizona, Texas, New York, Vermont, and other border states. They are allowed to do different things depending upon if the checkpoint is fixed or roving.

For example, there is a fixed checkpoint at just about the 100-mile point in I-5 in California near Camp Pendleton. When operating, they make everyone slow down and border patrol agents look at you and your passengers. I've never been asked to declare my citizenship at this checkpoint, but this questioning routinely occurs at other checkpoints.

A court case defined 3-4 criteria that the border patrol must establish in order to conduct a warrantless and suspicionless search of you and your vehicle/possessions. The one I remember specifically is that they must establish that "most" of the vehicles passing through the checkpoint can be assumed tohave crossed the border and have maintained continuous travel up to the checkpoint. For the one in California on I-5, this criterion doesn't even pass the giggle test, but, the checkpoint is allowed to persist.

3. Yes, the border patrol could set up one of these checkpoints right smack in the middle of downtown Detroit if they wanted to. (I think they actually tried in the recent past.)

Personally, I would love to see a high enough court declare these internal checkpoints illegal and unconstitutional. The government has absolutely no right to demand that I declare my citizenship inside my own country when I am not attempting to enter it at a border -- actual or at an airport.

As we have seen at airports with the TSA using their checkpoint as a dragnet for all sorts of things, the border patrol and local/state cops have used these internal checkpoints for all sorts of warrantless searches, including suspicionless searches for drugs.

There is a guy who works at a university observatory in southern Arizona who has challenged these checkpoints and currently has a court case going on (I think it's still active). He regularly videotapes his encounters at one checkpoint. His face-off with a female border patrol agent asking him to declare his citizenship is a classic -- and pretty gutsy. I will admit his website promotes his point of view, but he has done a lot of research on this subject:

Checkpoint USA Site

Click on the "Roadblock Revelations" link which will take you to his blog. The encounter with the female agent is on the second page of the blog -- Jan 2008, I think.