We can share the internet connection using airport, Here is the procedure:

1) Go to system preferences->sharing->internet sharing (a tab in Tiger or it is in the list in Leopard)
2) Under the pull down labeled "Share you connection from" choose ethernet. In the list below check the airport box. Click start.

If you have custom ipfw policy, need to add following 2 rules:

ipfw add allow ip from any to any via wlt*
ipfw add allow ip from 10.0.0.0/8 to any

You need to make sure the wireless network is accessable only for the trusted computer!