Well, I was hoping of a more general solution, like having a log of everything that any X client dumps to the console on a given session (a /var/log/messages alike for X).
If X crashes I just can't go out and grab some client's console... (I don't even know the culprit then).
I founf it out, GDM redirects the output to "/tmp/xses-${USER}.${SOME_RANDOM_CHARS}"
Carlos Rodrigues