$ defaults read com.apple.mail UserHeaders
$ defaults write com.apple.mail UserHeaders \
'{"Disposition-Notification-To" = "user@domain"; }'
Note that using the second command will erase all your other custom headers (the first command shows any you have created). So it might be easier to use the Property List Editor if you want several custom headers, such as a FOAF link. The .plist is located at ~/Library/Preferences/com.apple.mail.plist. With that done, it is just a matter of adding a rule to Mail.app to handle incoming Read Receipt requests. You'll need to use the "Edit Header List..." option to add Disposition-Notification-To to your list of choices. My final rule looks like this:
- Description: Read Receipt Requested
- if (all) of the following conditions are met:
- (Disposition-Notification-To) (contains) (@)
- Perform the following actions:
- (Run AppleScript) (~/Library/Scripts/Mail Scripts/Return Receipt.scpt)
[robg adds: I haven't tested this one.]

