http://www.innocenceproject.org/know/
I'm of the opinion that it's basically impossible to know how many wrongful convictions have taken place, but those 266 cases are surely the tip of the iceberg when you consider the difficulty in identifying the cases, determining if potential exculpatory evidence existed, testing that evidence and then pushing all of that through a courtroom.
But were they really innocent?
DNA evidence can be misused in the courts:
http://www.newscientist.com/article/mg20727743.300-how-dna-e...
No.
The article is very clearly questions the role of DNA evidence as a whole.
A couple of relevant quotes:
"Defence experts were trusting that the scientists had interpreted the data correctly. This perpetuated the myth that DNA is infallible."
"There's a considerable lack of understanding, not just from the public, but from the judges and lawyers."
This seems to be the author's opinion about what the article is about.
Make of it what you will. The article speaks for itself.
edit: BTW, it's a very interesting article - thanks for pointing it out.