Comment 1 for bug 750684

What would the tray icon do?

Either an icon "no new mail" or "new mail".
Show a tooltip "4 new messages".
And a menu "open inbox", "compose message".

Personally I find tray icons horrible from a usability standpoint, as opposed to indicators and launchers in a dock, both of which we support. That said, no definite decision has been made on this. If lots of people find it useful, it's a possibility.