Comment 5 for bug 742530

To my surprise, it looks like this is fixed for Oneiric.

I unfortunately can't provide a screenshot and to reproduce you will need at least two monitors as well as a Natty system, but on Natty it used to behave as described above: The Unity panel's emulation of the old GNOME2 notification area wasn't shown on all monitors (even if there were multiple panels), but only on one.
Which monitor that one was, iirc changed from "the most left one" to "the primary one". But I'm absolutely not sure about the last point, might also be that still the most left one was used.