Is it okay to collect DNA from criminal suspects on the sly?

I'm not sure that it's a violation of one's privacy to have a DNA sample collected on the sly. How is this any different that collecting anything else that a person might leave behind?

They collect fingerprints, clothing fibers, and everything else during a criminal investigation, so why not DNA if it gets left behind?

Apparently this practice has caused controversy in the courtroom. From the article:

"The practice, known among law enforcement officials as "surreptitious sampling," is growing in popularity even as defense lawyers and civil liberties advocates argue that it violates a constitutional right to privacy.

[...]Critics argue that by covertly collecting DNA contained in the minute amounts of saliva, sweat and skin that everyone sheds in the course of daily life, police officers are exploiting an unforeseen loophole in the requirement to show "probable cause" that a suspect has committed a crime before conducting a search." Source: New York Times

So what do you think?