The paper Jed cited is excellent. This problem is not going away and as David suggested it may the thing that drives us from Afghanistan sooner rather than later. The paper states that these killings occur mostly because the Afghans murderers are simply angry with the behavior of the ISAF. These are not Taliban agents doing the killing.

Another thing I read is the figures released by the ISAF severely understate the problem because they only report the deaths (I forget about wounded). They don't report the unsuccessful attempts where the would be killers miss or are killed before they can do the deed.