Most, if not all, the user interface updated correctly when I went from Natty to Oneiric but the system button did not. How can I get rid of the ugly purple monitor icon to be replaced with the new icon used by Oneiric? Is it a Unity-2D thing?
In general your icon theme looks outdated. Open the system settings and change the theme to high contrast, then back to Ambiance.
If that doesn't fix it, then run:
If the theme does not automatically update, change the theme as mentioned above.