xrandr 1.2 and ati 6.8: external DVI monitor stays blank

[Frederik]

I'm using an Apple Powerbook G4 with ATI Radeon graphics card and DVI 
output on Debian Lenny/Sid. When I try to enable the DVI output with 
xrandr, it seems like everything works from a software point of view (I 
see in GNOME desktop applet that my desktop stretches to the new size), 
but the external DVI monitor remains blank.

xrandr -q shows that the external DVI monitor is recognized fine:

Screen 0: minimum 320 x 200, current 2960 x 1050, maximum 3200 x 2048
LVDS connected 1280x854+0+0 (normal left inverted right x axis y axis) 
321mm x 214mm
   1280x854       60.0*+
   1280x800       60.0  
   1280x768       60.0  
   1024x768       60.0  
   800x600        60.3  
   640x480        59.9  
DVI-0 connected 1680x1050+1280+0 (normal left inverted right x axis y 
axis) 459mm x 296mm
   1680x1050      59.9*+   60.0  
   1280x1024      75.0     59.9  
   1280x960       59.9  
   1152x864       75.0     74.8  
   1024x768       75.1     70.1     60.0  
   832x624        74.6  
   800x600        72.2     75.0     60.3     56.2  
   640x480        75.0     72.8     66.7     60.0  
   720x400        70.1  
S-video disconnected (normal left inverted right x axis y axis)

http://artipc10.vub.ac.be/xorg/xorg.conf
http://artipc10.vub.ac.be/xorg/Xorg.0.log

$ dpkg -l "*xserver*" | grep -E "^ii"
ii  x11-xserver-utils                     7.3+2                        X server utilities
ii  xserver-xorg                          1:7.2-5                      the X.Org X server
ii  xserver-xorg-core                     2:1.4.1~git20080131-3        Xorg X server - core server
ii  xserver-xorg-input-all                1:7.3+10                     the X.Org X server -- input driver metapacka
ii  xserver-xorg-input-evdev              1:1.2.0-1                    X.Org X server -- evdev input driver
ii  xserver-xorg-input-kbd                1:1.2.2-3                    X.Org X server -- keyboard input driver
ii  xserver-xorg-input-mouse              1:1.2.3-2                    X.Org X server -- mouse input driver
ii  xserver-xorg-input-synaptics          0.14.7~git20070706-2         Synaptics TouchPad driver for X.Org/XFree86 
ii  xserver-xorg-input-wacom              0.7.9.3-2                    X.Org X server -- Wacom input driver
ii  xserver-xorg-video-all                1:7.3+10                     the X.Org X server -- output driver metapack
ii  xserver-xorg-video-ati                1:6.8.0-1                    X.Org X server -- ATI display driver
ii  xserver-xorg-video-chips              1:1.1.1-9                    X.Org X server -- Chips display driver
ii  xserver-xorg-video-fbdev              1:0.3.1-4                    X.Org X server -- fbdev display driver
ii  xserver-xorg-video-glint              1:1.1.1-8                    X.Org X server -- Glint display driver
ii  xserver-xorg-video-imstt              1:1.1.0-7                    X.Org X server -- IMSTT display driver
ii  xserver-xorg-video-mga                1:1.4.8.dfsg.1-1             X.Org X server -- MGA display driver
ii  xserver-xorg-video-nv                 1:2.1.8-3                    X.Org X server -- NV display driver
ii  xserver-xorg-video-s3                 1:0.5.0-4                    X.Org X server -- legacy S3 display driver
ii  xserver-xorg-video-s3virge            1:1.9.1-7                    X.Org X server -- S3 ViRGE display driver
ii  xserver-xorg-video-savage             1:2.1.3-5                    X.Org X server -- Savage display driver
ii  xserver-xorg-video-sis                1:0.9.3-6                    X.Org X server -- SiS display driver
ii  xserver-xorg-video-sisusb             1:0.8.1-9                    X.Org X server -- SiS USB display driver
ii  xserver-xorg-video-tdfx               1:1.3.0-6                    X.Org X server -- tdfx display driver
ii  xserver-xorg-video-trident            1:1.2.4-1                    X.Org X server -- Trident display driver
ii  xserver-xorg-video-v4l                0.1.1-6                      X.Org X server -- Video 4 Linux display driv
ii  xserver-xorg-video-vga                1:4.1.0-8                    X.Org X server -- VGA display driver

Am I doing something wrong, or does this appear to be a bug?

-- 
Frederik