In a 5-4 decision, the US Supreme Court has ruled in Maryland v. King that the police can collect DNA samples from people arrested for "serious crimes." The New York Times reports:
The federal government and 28 states authorize the practice, and law enforcement officials say it is a valuable tool for investigating unsolved crimes. But the court said the testing was justified by a different reason: to identify the suspect in custody.
“When officers make an arrest supported by probable cause to hold for a serious offense and they bring the suspect to the station to be detained in custody,” Justice Anthony M. Kennedy wrote for the majority, “taking and analyzing a cheek swab of the arrestee’s DNA is, like fingerprinting and photographing, a legitimate police booking procedure that is reasonable under the Fourth Amendment.”
Justice Antonin Scalia summarized his dissent from the bench, a rare move signaling deep disagreement. He accused the majority of an unsuccessful sleight of hand, one that “taxes the credulity of the credulous.” The point of DNA testing as it is actually practiced, he said, is to solve cold cases, not to identify the suspect in custody.
But the Fourth Amendment forbids searches without reasonable suspicion to gather evidence about an unrelated crime, he said, a point the majority did not dispute. “Make no mistake about it: because of today’s decision, your DNA can be taken and entered into a national database if you are ever arrested, rightly or wrongly, and for whatever reason,” Justice Scalia said from the bench.
I think Justice Scalia is correct. The first thing law enforcement will do is lower the definition of "serious crime" as low as possible so that they can DNA sample as many people as possible. The DNA will remain on file in some database even if the case is dismissed, just like records of arrests often linger for the rest of a person's life.