The Dead and the Damned
That's not entirely correct. A more accurate phrasing would be"we don't know if we've ever executed an innocent person."
That's because once someone's executed for a crime, prosecutors in most jurisdictions close the books on the case. Permanently. Files are generally either sealed or destroyed. The reason? Precisely so arguments like Goldberg's retain their validity. No one wants to be the first DA who's proven to have sent an innocent man to his death. Once the convicted is put to death, odds are no one will ever lay eyes on the kind of evidence that might later point to doubt. The files are gone. Or inaccessible.
A second reason we'll never know is because public interest groups who take capital cases -- Barry Scheck's"Project Innocence," for example -- aren't ever likely to spend precious resources clearing the name of a dead man when there are hundreds of cases of the living accused in the queue. Yes, it would certainaly give death penalty opponents a great rhetorical tool if they could prove the execution of an innocent, but if you've only got the money and time to take ten cases, and you're looking at hundreds of potential living innocents behind bars, it's hard to justify taking on a dead client.
Goldberg may or may not be right (I'd strongly suspect he isn't). The point is, we'll really never know.
Check here for Goldberg's response to this post, and my reply to his response.