This bug is http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=540521 passed upstream - there is plenty of detail of all manner of probably-mostly-irrelevant things in that. I'll summarize the important points:

With an xorg.conf of,

Section "InputDevice"
        Identifier      "Generic Keyboard"
        Driver          "kbd"
        Option          "XkbRules"      "xorg"
        Option          "XkbModel"      "pc104"
        Option          "XkbLayout"     "us"
EndSection

Section "InputDevice"
        Identifier      "Configured Mouse"
        Driver          "mouse"
EndSection

Section "Device"
        Identifier      "Intel 945GM"
        Driver          "intel"
EndSection

Section "Monitor"
        Identifier      "Configured Monitor"
        Option          "DPMS"
EndSection

Section "Screen"
        Identifier      "Default Screen"
        Monitor         "Configured Monitor"
        SubSection "Display"
           Depth                24
           Virtual              3120 1056
        EndSubSection
EndSection

everything was working fine for me. I had dual-heading working with,

~$ xrandr --query
Screen 0: minimum 320 x 200, current 3120 x 1050, maximum 3120 x 1056
VGA connected 1680x1050+0+0 (normal left inverted right x axis y axis) 473mm x 296mm
   1680x1050      60.0*+   60.0     60.0* 
...
LVDS connected 1440x900+1680+0 (normal left inverted right x axis y axis) 370mm x 230mm
   1440x900       59.9*+
...
TV disconnected (normal left inverted right x axis y axis)
~$

However, since upgrading, both displays are forced down to 1440x900 and dual-heading won't work, to wit,

~$ xrandr --output VGA --left-of LVDS
xrandr: screen cannot be larger than 2048x2048 (desired size 2880x900)

I tried disabling acceleration by adding to my xorg.conf,

Section "Module"
        Disable "dri"
        Disable "dri2"
        Disable "glx"

        SubSection "intel"
          Option "DRI" "false"
        EndSubSection
EndSection

but that doesn't help. Brice Goglin suggests that being able to disable UXA could help.

I shall now attach my Xorg.0.log files.