Sometimes happen the same situation: one ethernet port, one ethernet cable, no hub/switch and many PCs with wireless.
A good solution is to connect a pc with ethernet and create a WLAN (ad hoc or infrastructure) to connect other PCs to internet.
Too many time I had this problem and the solution is always the same or similar, so I wrote this useful and reusable bash script to configure wifi card (chipset Atheros with madwifi driver) and create a NAT to connect AdHoc WLAN.


#!/bin/bash
# inizializing ethernet (I suppose outer network have address 193.205.22.12)
# I suggest to disable NetworkManager and kill dhclient
ifconfig eth0 193.205.22.12


# Now I need to unload and reload module with option "autocreate=adhoc". This
# simplify the creation of virttual interfaces athX (See how madwifi work for more informations)
/sbin/rmmod ath_pci
modprobe ath_pci autocreate=adhoc

# Configuring essid (In this case I use essid "spongepowa"
iwconfig ath0 essid spongepowa
# Configuring WLAN address (I suppose my network is 192.168.1.0/24)
ifconfig ath0 192.168.1.1


# Enabling forwarding
/bin/echo "1" > /proc/sys/net/ipv4/ip_forward


# Inizializing iptables
iptables --flush
iptables -t nat -P POSTROUTING ACCEPT
iptables -t nat -P PREROUTING ACCEPT

# Making rules:
# - 192.168.1.0/24 is local WLAN addresses
# - eth0 is the output interface (ethernet interface), ath0 is the input interface (wifi interface)
iptables -A FORWARD -s 192.168.1.0/24 -d 0/0 -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -d 192.168.1.0/24 -j ACCEPT
iptables -t nat -A POSTROUTING -o eth0 -s 192.168.1.0/24 -j MASQUERADE

I hope this is correct. If you have any question ask me by comment.

If my english is full of mistakes, tell me (AYBABTU is not allowed without motivation :þ)…with my mistakes, of course.