If /root is full, xdm tries to spawn the X server several times for a minute or so, then dies dumping you into the login prompt. There is no message that the /root partition is full. XFS won't start as well. In this failure mode, for newbies, there SHOULD be a message or indication of the problem, and it should not take a minute or so while the X server blinks and tries to startup. I have duplicated this on both of my Toshiba Tecra 8000 laptops (neomagic driver).
This is a linux (or even a unix) problem rather than an XFree86 problem. A fix to avoid the problem in the future is to partition your disk so that root almost never runs out of space -- this usually means putting /tmp, /var, and /usr on other partitions.
Yeah still ... I've had the problem. And only after 2 reinstalls I noticed that the root was full. I agree that their should be some sort of messages about this.