...well, some of them are waiting.
Others are already there.
The concept's pretty simple. Network your XBox to a computer, which connects to other computers via a broadband connection (DSL or Cable Modem), which in turn connects to other XBoxes. Your console believes it is connected directly to the other players via system link cable, so any game which supports this feature (Halo, Tony Hawk 2X, NASCAR Heat 2002) will work.
- XBoxGW is reportedly the fastest and most reliable solution. They walk you through creating a boot disk which starts your system running in Linux with appropriate scripts loaded. If you know your opponents' IP address, you're good to go. Otherwise, that's going to be a problem for you.
- If you want your computer to remain active while you're playing, switching into Linux isn't your best option. You'd likely prefer the VPN route. This entails configuring your Windows (although you might be able to extrapolate this info for MacOS) network to allow proper connections to and from your XBox. Pretty straightforward, but slower than XBoxGW, and you'll still need your friends' IP address if you'd like to connect.
- The most user-friendly solution, though probably slowest, is Gamespy Arcade's XBox Tunnel. Gamespy will find you opponents to play against, which is a definite plus. I don't believe there's a Mac version, but Apple's following is openly anti-Microsoft, so I don't know what the demand is anyway.