This happens when I change the screen with:
xrandr --output VGA --off --output LVDS --auto
So I move the output from the external screen to the laptop's panel.
I'm using the driver version 1.2.0, with the latest related stuff (and the latest libXinerama 1.0.2). I can reproduce it on at least some applications everytime I switch from the VGA screen to the flat panel (and more rarely when I switch the other way around). The VGA screen is bigger (1440x900 vs 1024x768).

Here is a stack trace from a sample GTK+ app (the gnome panel, but all the gtk+ apps crash too). and some other data from my gdb debugging session.

(gdb) bt
#0  init_xinerama_support (screen=<value optimized out>) at gdkscreen-x11.c:617
#1  0xb76c9b78 in _gdk_x11_screen_size_changed (screen=0x80d90f8, event=0xbfb14f5c)
    at gdkscreen-x11.c:753
#2  0xb76bcaf3 in gdk_event_translate (display=0x80d60e8, event=0x81a6198, 
    xevent=0xbfb14f5c, return_exposes=0) at gdkevents-x11.c:1825
#3  0xb76be1c9 in _gdk_events_queue (display=0x80d60e8) at gdkevents-x11.c:2252
#4  0xb76be545 in gdk_event_dispatch (source=0x80dd400, callback=0, user_data=0x0)
    at gdkevents-x11.c:2312
#5  0xb7285183 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0

(gdb) p screen_x11->num_monitors
$3 = 1
(gdb) p monitors
$6 = (int *) 0x80e6030
(gdb) p monitors[0]
$7 = 1
(gdb) p *((XineramaScreenInfo*)monitors)
$14 = {screen_number = 1, x_org = 0, y_org = 0, width = 0, height = 0}