Gmail app: How can I view the email addresses?
The latest version of the Gmail app (it has not been released yet, but you can find a copy of a leaked official APK floating around somewhere) has this feature. It adds a "more details" link on every email header so you can see the actual email addresses used.

I'm not sure whether this version will work on 1.5 and up, as Google hasn't said anything yet. I know that part of their work in 2.2 was better separating the Google apps like Gmail from the rest of the platform, so that they could ship updates through the Market. So it's possible that this will only become available for 2.2.