The short answer? Yes. The iPhone will tether, and not just the 3G iPhone, the 1st gen iPhone will tether too.
Setting up tethering with the iPhone is a bit technical, but I think you can handle it. The instructions below don't mention creating a new location in the network settings for this purpose, but I highly recommend it. I have several locations, each with a specific use. For example, I have a home location that uses a static IP to get around the Leopard DNS issues (pokey lookups). I have several locations with static IPs for the same reason, actually. Anyway, I recommend that you set up a new location with just the AirPort interface in it, named something creative like “Tether.” When you want to tether, switch to that location, and if you have problems you'll be assured that other interface settings aren't interfering.
The application is $9.99 and can be found here: NetShare. Appropriately, some of the commenters in the App store suggest that you keep an eye on your usage as AT&T has a ceiling on the “unlimited” bandwidth plan.
The simple instructions for Mac users on Leopard are here: Macrumors.
Let me know if you need help. I'll do what I can.