The last launchpad rollout fixed bug 111147, no longer exposing the email address of people who have hidden them in the launchpad interface. The unfortunate side effect of this is that there's now no programmatic way to go from an emailed comment back to the either the identity of the commenter or the specific comment on the bug report.
To give some context, I prepare a weekly thank you report of people who test out proposed fixes in Ubuntu. I give this report in both an irc and wiki context (for an example wiki report, see https://wiki.ubuntu.com/QATeam/SRUReports/2010-07-07), and I attempt to use the registered irc nicknames and wiki names when doing so.
Because the reported testing comments are free-form and are not identifiable programmatically, I do this by manually sifting through comments on the associated bug reports. For a week's report, this can involve examining a couple of hundred bug reports, thus it's far less time consuming to examine unread comments in an email client rather than opening up a couple of hundred bug web pages and scrolling to view the most recent comment (we have a specific team subscribed to those reports where a fix is proposed, and I filter email sent to that team). This workflow was possible because the commenter's email address was given as the From: address, and from that their irc nick and wiki name (if registered) could be looked up via the launchpad.people.findPerson(text=email) launchpadlib api, even for people who's email addresses were hidden in launchpad. (In hindsight, I see I should have been using launchpad.people.findByEmail().) None of the many X-Launchpad headers in an email message allow you to figure out the launchpad id of the person sending the comment.
I think protecting people's email addresses is important, particularly when we've told them that it is hidden, so I'm not asking that the fix for bug 111147 be reverted. Rather, I'd like to see a header added that (a) either gives the launchpad id of the commenter directly (note that *all* the commenters on a bug are already included in the X-Launchpad-Bug-Commenters: header, but it doesn't distinguish who *this* commenter is) or (b) identify which comment number the email is associated with, so that the launchpad id can be lookup from the specific bug.message's owner field.
I'm working on fixing notifications at the moment. One way that we can fix your problem is with an X-Launchpad-