Upgrading to Gnome3 PPA on Ubuntu 11.04 had a very unstable behaviour ( Screen flickerings, Nautilus without theme icons etc ..). This occured cause upgrading to Gnome 3 on Natty breaks the default theming used by Ubuntu Classic & the new Unity desktop environments by setting the icon-theme to gnome and the gtk-theme to Adwaita.

Gnome 3 ignores the previous gconf interface settings and only reads the dconf ones.
You can set it right by editing the dconf file using dconf-editor or try the steps below.

Few things as workaround that can be done before GNOME 3 gets a permanent fix, the below steps worked for me hope it helps you too.

    1. Remove gnome-accessibility-themes and install gnome-themes-standard
    sudo apt-get remove gnome-accessibility-themes
    sudo apt-get install gnome-themes-standard

    2. Edit /usr/share/xsessions/gnome.desktop
    change the line Exec=gnome-session –session=ubuntu to
    Exec=gnome-session –session=gnome

below are the content of gnome.desktop file

    [Desktop Entry]
    Comment=This session logs you into Ubuntu
    Exec=gnome-session –session=gnome

    3. Install gnome-tweak-tool
    go to interface tab, set gnome theme to “ADWAITA” and
    icon theme to “unity-icon-theme” and reboot