I've noticed that strange things start to happen if network connection is down. Before getting a new router/adsl modem gdm wouldn't let me login to Gnome session. Windowmaker worked flawlessly. Similar thing happened on Knoppix installed on another machine: if network cable was unplugged, kdm would not start.
The problem was (if I recall correctly) that the "Fully Qualified Domain Name" could not be resolved. What it means, is that it tries to get hostname information from the network, but it just can't, so it halts.
I fixed the thing by following command:
dpkg-reconfigure etherconf # if necessary, apt-get etherconf
Hostname = localhost
Domain = # leave empty
eth0 = not removable, configure with dhcp, no dhcp-hostname
Previously I had used my machine's network name and my university's domain as hostname and domain.