My roommate and I share a cable modem, since I haven't gotten around to configuring a firewall/gateway box for our local network. We just unplug one machine from the back of the cable modem and plug in the other. I've noticed that when I'm unplugged, as if I were still online, when I type 'startx', it stalls for about 2 minutes, then finally kicks in and starts X like normal. When I look at the output of 'startx', I notice that there is a line something like: 'xauth: cannot determine hostname'. My hostname is the one given me by my cable modem provider.
This is a DNS misconfiguration problem, not an X bug. Please configure your /etc/hosts and /etc/resolv.conf properly to avoid this problem with X, and with other network services as well.