[TheMercenary]

Under the 4th Amendment exceptions to detainment can be made...

Exceptions
The government may not detain an individual even momentarily without reasonable and articulable suspicion, with a few exceptions.

Where society's need is great and no other effective means of meeting the need is available, and intrusion on people's privacy is minimal, checkpoints toward that end may briefly detain motorists. In Michigan v. Sitz 496 U.S. 444 (1990), the Supreme Court allowed discretionless sobriety checkpoints. In United States v. Martinez-Fuerte 428 U.S. 543 (1976), the Supreme Court allowed discretionless immigration checkpoints. In Delaware v. Prouse 440 U.S. 648 (1979), the Supreme Court allowed discretionless checkpoints for driver's licenses and registration. In Illinois v. Lidster 540 U.S. 419 (2004), the Supreme Court allowed focused informational checkpoints. However, discretionary checkpoints or general crime-fighting checkpoints are not allowed.[29]

Another exception is at borders and ports of entry.

Roadblocks may be used to capture a particular fleeing criminal or locate a bomb.

http://en.wikipedia.org/wiki/Fourth_...s_Constitution

And here the courts found that stopping people who look like they are of Mexican ancestory is legal:

We further believe that it is constitutional to refer motorists selectively to the secondary inspection area at the San Clemente checkpoint on the basis of criteria that would not sustain a roving-patrol stop. Thus, even if it be assumed that such referrals are made largely on the basis of apparent Mexican ancestry, 16 we perceive no constitutional violation. Cf. United States v. Brignoni-Ponce, 422 U.S., at 885 -887. As the intrusion here is sufficiently minimal that no particularized reason need exist to justify it, we think it follows that the Border Patrol [428 U.S. 543, 564] officers must have wide discretion in selecting the motorists to be diverted for the brief questioning involved. 17

http://caselaw.lp.findlaw.com/script...=428&invol=543