I think it would be good to get away from the heavy, grayish appearance of the message header under Win7. It's a bit hard to tell where to get inspiration for a new one however, since few Win applications have a message header :) The info pane from the file manager (attached) might serve as inspiration in this case. There is also the header from Windows Live Mail http://www.activewin.com/screenshots/windows7/Windows%20Live%20Mail.png , but I'm not sure I'm a fan of the shadow coming out of nowhere there, even though it looks slightly better than when applied to the sidebar :)

This is being implemented as part of bug 569400