Comment 7 for bug 274461

indeed, it seems that it's a design desicion rather than a bug http://bugzilla.gnome.org/show_bug.cgi?id=515907 added RGBA to system monitor so it can use the transparency capabilities of the Murrine GTK+ theme and yep it's only showed with a composite manager in this case compiz, you can probably also see it with metacity with the composite manager enabled, not a bug but a design desicion.