applekid's unofficial guide to using your Mac as a bridge for the Xbox 360 v. 1.0
Disclaimer: Your mileage may vary. This guide is not guaranteed to work, but hopefully people here can help me edit this to make it better. I'm not a networking guru so some of my guesses in throughout the guide may be incorrect.
For starters, make sure your router is up and running, your Mac is connected to the router and the Internet, and there is an ethernet cable running between your Mac and Xbox 360.
Setting up your router
The following ports need to be forwarded to your Mac (so find the IP address in the Network pane of System Preferences under the TCP/IP tab) for Xbox Live to function:
-UDP 88
-UDP 3074
-TCP 3074
source http://support.microsoft.com/kb/908874
If you are somewhat network savvy, you can use a DMZ server instead. For more on that, please use Google. I attempted using it, but I didn't know if I was even doing it right. And in the case of using a Mac as a bridge, I think that leaves your Mac vulnerable to attack, since I believe you have to set the Mac as the device that will be in the DMZ.
Setting up the Mac
First, let's go into the Network pane in System Preferences. Make sure under Network Status you have a Built-in Ethernet connection option. Open it up. Configure IPv4 should Using DHCP. It's just important that you haven't disabled your Ethernet port.
Go into the Sharing pane in System Preferences. Choose the Firewall tab. Make sure the firewall is off. Yes, that's right. Off. Your Mac should not be vulnerable to any attack, assuming your router has a firewall.
If you're feeling paranoid, you can open up the mentioned ports in the router section and leave your firewall on, but you can't access Windows Live Messenger or the Video Marketplace. It seemed like I had some lag issues here and there in certain online games with my Mac's firewall on. And, I think this will block your ability to give/receive invitations for games/chats. Not confirmed because all of my friends have some kind of issue with Xbox Live in one way or another...
Setting up the Xbox 360
The fun part. Go to the System blade and select Network Settings and choose Edit Settings.
Start with the first section. Choose Manual for IP Settings.
IP Address: 192.168.2.50 (I think anything that isn't your Mac's IP address is fine... Or maybe anything connected to the router? Lack of networking knowledge...)
Subnet Mask: 255.255.255.0
Gateway: 192.168.2.1
Choose done.
Go to the second section. Choose Manual for your DNS Settings.
I have no idea how important the DNS settings are, but I put in:
Primary DNS Server: 192.168.2.1
Secondary DNS Server: 192.168.1.1
Choose done.
Now back out to the Network Settings menu. Test out your Xbox Live Connection using the Test Xbox Live Connection option in the Network Settings pane. Hopefully things go well here. Everything should be Confirmed. Depending on your luck, you might get Open or Strict for NAT. I get Strict for reasons beyond me.
My only failure with Live has been receiving/giving invites to friends for games/chats. I had one friend that was able to invite me to a chat successfully, but I didn't have my headset hooked up, so I'm not sure if that's 100% working. However, if it wasn't working, I would've gotten some kind of error or been kicked out of the chat instantaneously, so I believe everything is working on my setup. I probably should really test it out with some MR members...
Other sources:
http://www.macosxhints.com/article.php?story=20041112101646643&query=xbox+live
http://meyerweb.com/eric/thoughts/2007/04/26/xbox-live-via-a-mac-laptop/
http://www.tuaw.com/2007/03/26/how-to-using-your-mac-as-a-nat-router/
http://www.xbox.com/en-US/support/connecttolive/xbox360/homenetworking/equipmaent.htm
http://www.tuaw.com/2007/04/16/tuaw-guide-xbox-360-and-mac/
Feel free to comment and complain. I'm doing this partly from memory, partly from looking at the settings on my router, Xbox, and System Preferences, and finding links that I think I used.