The bug has been opened on https://bugs.launchpad.net/ubuntu/+source/gdm/+bug/116392

"Binary package hint: gdm

gdm, as of updating gutsy to the latest, is crashing on me. The stack trace of the crash is:
...
Interesting. Here's the new stacktrace:

(gdb) where

+ Trace 136328

And here's the problem:

 if (SERVER_IS_LOCAL (d) && ! added_lo && ! d->tcp_disallowed) {
  struct sockaddr_storage *lo_ss = NULL;
  /* FIXME: get loobback ss */
  if (! add_auth_entry_for_addr (d, &auths, lo_ss)) {
   goto get_local_auth_error;
  }
 }

in get_local_auths(). lo_ss == NULL is passed to add_auth_entry_for_addr() which doesn't even test it for NULL and starts using it and ends up assigning one if it's members to "addr" which ends up being used in a memcpy. Bad.
...
I was just about to dig in and get to the root of this problem and saw didier's post. Indeed, disabling TCP solves my crash too. I do like my X server listening on the TCP socket though.
..."