Cory,
Britt Blaser turned me onto your question since it is right in line with the kinds of things I investigate and answer for people on a new service of my Mac Net Journal site called AskMNJ? I did a little digging around and found:

1) Apple's docs say you cannot be connected to a wireless network and then share that connection with others using the Software Base Station built into OS X 10.2.x.

2) It looks like you could however use IPNetShareX (free) or its shareware cousin IPNetShareX Pro ($25) from Sustainable Networks, the same folks who make the IPNetRouter software you used for this kind of situation under Mac OS 9.x. IPNetShareX provides a GUI for access the gNat capabilities of Mac OS X. Of course, I am sure you could dig around in OS X and find the Unix commands to work directly with gNat.

3) It would probably also work to buy a second Wi-Fi PC Card to give a second interface to help share the connection.

All of these assume that what you are trying to do is, for instance, to be able to go to Starbucks and log onto your T-Mobile account and then share that wireless connection with your friend who doesn't already have his own T-Mobile account set up. Is this what you are talking about?

Really? I thought you needed two Wifi cards for this to work...

The supercool PDOS people are building something called Grid which makes the network work like this by default. You throw the packets and your nearest neighbor, who passes it on. They have drivers for Linux; maybe some enterprising hacker can port them to OS X.

Deploying something like this is the key to making large wireless networks, and reclaiming the routers for ourselves.

Grid

If you're picking up a signal, and your iBook is Airport-equipped, go to the Airport menu item and choose Create Network. Make a network, and name it. Now, as long as your iBook is on the network, other computers will be able to access it wirelessly through you.

Or, at least, it works for me. We've had a chain of wireless connections on our science and technology building here (here being NKU) through this feature.