OK, I have to say the new Xorg 6.9 on 3.9 beta seems to be a little more stable.
The most workable config I have found is to use xdm as I have described here, with this config it properly switches back to xdm login if for some reason you do overload the X server (mouse event queues too long with lots of paging activity seem to do this). This is nice since when the X server dies ungracefully there is no switch back to text mode emulation at that vt, at least it allows for recovery by logging in again.
So, I'm getting closer to my 'holy grail' of long 'uptime' figures
- Andy