Comment 61 for bug 974480

I also miss this functionality and would prefer this to be available out of the box in Ubuntu.

Meanwhile I found that "stalonetray" works very reliably as a replacement.
You can also try "trayer".

Note that stalonetray requires some configuration, at least put the following in ~/.stalonetrayrc
window_layer top
geometry 4x2+660+0
grow_gravity NW
icon_gravity NE

Trayer is less configurable but works out of the box.

Hope this helps