Comment 4 for bug 726124

Sort of. Gnome Shell supports extensions written in GJS (Gnome Javascript with introspection of Gnome libs) and uses those to unify some things. For example libnotify, Telepathy and Rhythmbox are handled by extentensions that use similar pop-ups (libnotify just message, Telepathy pop-ups allow replying to messages, Rhythmbox changing songs, etc).

However the 'panel' in Gnome Shell is more static than Gnome2 and doesn't support gnome-panel applets like indicator-applet, but just provides a 'status tray' that will hold GtkStatusIcon's. A Gnome Shell extension could probably be written for Postler, but since Gnome Shell development is still moving fast I just think a simple old-school status icon would do most of what is needed. Maybe one day I'll think about writing an indicator-applet wrapper in GJS.