Booting with console=ttyS0 and gdm/xorg strangeness

[Tim Day <timday@bottlenose.demon.co.uk>]

In order to try and diagnose some rare system freezes which seem to have
started following upgrade to etch, I'm booting the problem machine with
an added
  console=ttyS0,9600n8
boot option, and have another machine connected by a serial cable
monitoring it (I haven't changed any getty stuff; not interested in
logging in over serial).  This seems to work fine (in that I can capture
the console, although I haven't caught a crash yet).

However, when gdm comes up following the boot, I now can't type
anything, not even ctrl-alt-Fn to another virtual console.  (Mouse is
fine though).  But if I ssh in from another machine and restart gdm,
once it's comes up again it responds to the keyboard just fine and I can
proceed to a usable xsession as normal (and ctrl-alt-Fn to other
terminals).

I don't understand enough about the relationship between the kernel
console and X to know what's going on here.  Any ideas ?

Thanks for any info
Tim

---

FWIW, my xorg.conf contains:
Section "InputDevice"
        Identifier      "Generic Keyboard"
        Driver          "kbd"
        Option          "CoreKeyboard"
        Option          "XkbRules"      "xorg"
        Option          "XkbModel"      "pc104"
        Option          "XkbLayout"     "gb"
EndSection

[More background to this in the
  MSI nVidia NX7600GT-T2D256E
thread back ~28th August; it seems switching to "nv" from "nvidia" isn't
the answer to my problems as I've had two mystery hangs since then].