Adam Liptak has an article on a study of the convictions (now 200) reversed by DNA evidence in The New York Times. Liptak is a treasure, and this piece is based on the work of Brandon Garrett, a law professor at the University of Virginia.
I won't go into the substance of the article, which details a lot of misbehavior and some apparent racial factors in these wrongful convictions. Let me note, however, that the study's results suggest that there are thousands of other innocent people in prison--inmates who will not have the chance for vindication through DNA, because there is no biological evidence in their cases. (It is no accident that a very high proportion of those exonerated were convicted of rape.)
One of the most disturbing parts of the article is the way in which prosecutors have too often been prepared to accept flawed or sometimes fraudulent evidence. I don't mean to suggest that many of them connive knowingly at the use of such proof--although there are a disturbing number of instances in which that seems to be the case. Prosecutors seem to forget that their role is not merely to convict, but to serve justice. Perhaps it would help if they had this truth drummed into them: When the wrong person is convicted, the guilty person is still out there on the street.