From The Atlantic:
"The five Justices who supported the ruling — conservatives Roberts, Thomas, and Alito, along with Kennedy and the typically liberal Breyer — determined that such a system was akin to fingerprinting, with one exception. "The only difference between DNA analysis and fingerprint databases is the unparalleled accuracy DNA provides." Taking samples from those in custody, then, didn't comprise a suspect's Constitutional rights.
The dissent — representing the unusual coalition of Justices Scalia, Ginsburg, Sotomayor, and Kagan — strongly disagreed. "The Court’s assertion that DNA is being taken, not to solve crimes, but to identify those in the State’s custody, taxes the credulity of the credulous." And:
[T]he Court’s comparison of Maryland’s DNA searches to other techniques, such as fingerprinting, can seem apt only to those who know no more than today’s opinion has chosen to tell them about how those DNA searches actually work.
In other words, first, the dissent argues that this is not an identification tool but rather an investigative one, with which the four Justices take issue. Second, it argues that the complexity and involvement of DNA analysis makes it qualitatively different than fingerprinting.
Scalia's dissent ends with an admonition that may prove telling (and will certainly provide fodder to privacy advocates). Suggesting that the "serious offenses" distinction is a poor guidepost, he writes:
As an entirely predictable consequence of today’s decision, your DNA can be taken and entered into a national DNA database if you are ever arrested, rightly or wrongly, and for whatever reason.
If you're wondering, the FBI has maintained a database of fingerprints since 1924."