By default, the active and inactive tabs in gnome terminal and gedit aren't very different when one uses the Ambiance theme.
I posted a workaround a while ago here at the time of 11.10:
It still works for 12.04. In short, after backing up the file, edit /usr/share/themes/Ambiance/gtk-3.0/gtk-widgets.css and in the section titled ".notebook tab", replace
background-image: -gtk-gradient (linear, left top, left bottom,
from (shade (@bg_color, 0.97)),
color-stop (0.80, shade (@bg_color, 0.95)),
to (shade (@bg_color, 0.92)));
or whatever you like. Alternatively, just tweak the shade values in the existing code.
Please note that I'm using a laptop (Dell Inspiron 1545) and I don't know whether this will work on desktops or other devices.
Edit: this works with this Greybird theme (Xfce) installed on 12.04 as well. There, I just change .notebook tab and not .notebook tab:active.