I recently set up my Tiger Sever machine to serve (or share) its AirPort card as a base station, providing internet to anyone that connects to it. This was harder and more complicated than I originally thought, and I wish to share my experience.
First of all, there are two methods to share your AirPort connection. One is to manually set up (or use the new Gateway Setup Assistant) to configure NAT and DHCP and create a computer-to-computer network. The other is from the Internet Sharing (Sharing Preference Panel -> Sharing -> Internet) found in OS X Client (but hidden in Server). I will provide the benefits and disadvantages of both, as well as how to set them up from Tiger Server.
Method One:
For the first method, use the Gateway Setup Assistant (/Applications/Server) to share your internet connection to your AirPort Card. This only takes a few seconds and clicks. After that has been configured, create a computer-to-computer network with WEP if need be. The name of your computer-to-computer network will be the SSID that people will see in their wireless selection menus.
This works fine, except that when another OS X machine connects to your computer-to-computer network, they will have no wireless strength in their menubar, just a computer icon. Also, using this method is slow to provide a DHCP address, as a user's connecting computer will have a 10 to 15 second delay before getting its IP Information.
Method Two:
The second method is great if you don't need NAT or DHCP on the server for anything else. This method will allow you to use the Internet Sharing feature from OS X Client in OS X Server. Internet Sharing is still part of OS X Server, it is just hidden. To get to its settings, we need to trick the Sharing System Preference.
In /System/Library/CoreServices, there is a file called ServerVersion.plist. Make a copy of this file somewhere on your drive, and delete the original (you will need to enter the admin password). Now you may go into the Sharing System Preference, and click on Internet. Set up Internet Sharing to share your internet port to your AirPort, and enter a SSID -- and WEP encryption if you want. Now turn your AirPort on, and then start Internet Sharing. In the menubar, your AirPort icon should now have an arrow pointing up. This means your Internet is now being shared over AirPort.
When you have accomplished this, drag and drop the copy of ServerVersion.plist back into /System/Library/CoreServices, making sure its name is as it's shown here. If you don't put this file back, very bad things will happen to your system! This method will allow the user over AirPort to see his signal strength in the menubar, and have faster connection times compared to the standard method.
I really don't understand why Apple doesn't have a more robust solution for this, as it seems you can't disable SSID brodcasting, or set up MAC address filtering. Hopefully 10.5 server will address something simple like this.

