Positioning popup windows with Xinerama in dual monitor setup

[Guy Rouillier <guy-rouillier speakeasy net>]

I have an AMD64 laptop with the ATI 9600 Mobility chip.  To get the
native resolution on the built-in LCD, I had to install XFree86 4.4.0
from XFree86.org.  I now have independent resolutions on the LCD and on
an external monitor with Xinerama.

My problem is that whenever a window opens, it is positioned in the
"middle" of the two display devices, i.e., half is on the laptop and
half is on the CRT.  Someone told me that my window manager should
contain a command to move popup windows onto one monitor or the other. 
I've searched though the GNOME settings and can't find anything like
this.  Is there such a setting in gnome?  Thank you.

-- 
Guy Rouillier