K210.ORG

Ok, I finally found a way to get rid of that ugly white/black arrow near the Ubuntu [ or custom ] icon of menu button in GNOME.

sudo apt-get install apt-build
while configuring, choose your processor type
sudo apt-build source gnome-panel
sudo apt-get build-dep gnome-panel
cd /var/cache/apt-build/build/
sudo gedit gnome-panel-*/gnome-panel/panel-menu-button.c

now change the following part :
button = g_object_new (PANEL_TYPE_MENU_BUTTON,
"menu-path", menu_path,
"custom-icon", custom_icon,
"tooltip", tooltip,
"use-menu-path", use_menu_path,
"use-custom-icon", use_custom_icon,
"has-arrow", TRUE,
NULL);

into
button = g_object_new (PANEL_TYPE_MENU_BUTTON,
"menu-path", menu_path,
"custom-icon", custom_icon,
"tooltip", tooltip,
"use-menu-path", use_menu_path,
"use-custom-icon", use_custom_icon,
"has-arrow", FALSE,
NULL);

save & exit and

cd gnome-panel-*/
sudo ./configure --prefix=/usr
in case there are still some missing dependencies (build-dep may miss something), install them manually [ most of them are the -dev packages, so use synaptic for that - for example when it shows No package 'libgnomeui-2.0' found
, just search in Synaptic Package Manager for libgnomeui (forget about -2.0 or any other number) and install the -dev package, libgnomeui-dev.]
sudo make
sudo mv /usr/bin/gnome-panel /usr/bin/gnome-panel.with.arrow
sudo cp gnome-panel/gnome-panel /usr/bin/gnome-panel
sudo killall gnome-panel

that's all.