The Constituent History Report is not registering changes for the Do Not Email field.
We are currently evaluating this issue for a fix in a future service pack.
Steps to Duplicate
1. Open up any constituent record or create a new one. 2. Go to the Contact tab. 3. Edit an existing email address or add a new one. 4. Mark or unmark, depending on how the option is currently set, the "Do not send an email to this address" option. 5. Save the change. 6. Click on the History tab. 7. View the Constituent History Report.