There have been 123 death row exonerations since 1973 according to the Death Penalty Information Center. As of January 1, 2007, there were 3,350 people on death row in the U.S. That's 0.036% of cases.
Keep in mind that to qualify as "exonerated" under the DPIC's definition, a defendant must have been convicted, sentenced to death and subsequently either- a) their conviction was overturned AND i) they were acquited at re-trial or ii) all charges were dropped; b) they were given an absolute pardon by the governor based on new evidence of innocence. " http://www.deathpenaltyinfo.org/article.php?scid=6&did=110
A subsequent acquital does not mean the defendant was innocent -- it means the state couldn't prove the defendant's guilt beyond a reasonable doubt. That's why juries find people "not guilty" instead of "innocent."
There can be no doubt that there are innocent people on death row -- for the reasons Justin mentioned -- but the number is still probably very small. The fact of the matter is that many of the people on death row have done really horrific things and ought to be in prison (although a substantial portion were also just involved in run of the mill, contract enforcement-related drug murders).
Based on my own experience, I'd say the highest proportion of innocent people in prison are those convicted of sex crimes against children. Young kids are easily coached and manipulated -- and saying dad is a child molester is a good way to win a custody dispute. Even then, I suspect the proportion of innocent people who are convicted is only slightly higher than the average for other offenses.
-WD