No, you won't be able to run Xbox 360 games on your Mac - yet. However, this little hack may be handy if your XBox and your cable modem are in separate rooms,  and you don't want to shell out $100 for a wireless adapter.

Step 1:  turn off your Xbox 360 and connect a standard ethernet crossover cable from the network port of the 360 to the ethernet port on your Mac. On the Mac, go to System Preferences>Sharing.  Click the "Internet" tab.  On the "Share your connection from..." drop-down, select AirPort. Check Built-in Ethernet on the box underneath, then press 'Start'. You will get a cautionary dialogue box; accept it. Your window should look like this:

Step 2:  Turn on your 360, go to the dahboard and navigate to the System pane. Select "Network Settings" and then "Edit Settings". Change IP settings to Manual. Back in OS X, enter Terminal and type, without quotes: "ifconfig en0". The first IP address, which is probably 192.168.2.1, is your Gateway address.  Enter this on your Xbox. For IP address, enter 192.168.2.2, and 255.255.255.0 for the Subnet Mask.

Step 3: Save your settings on your Xbox 360. It  may automatically test the new connection; ignore this, because it will fail. Go back into the settings menu and this time choose DNS Settings, making this manual as well. Next, you'll need to go back into Terminal and type in "dig" (again, without quotes) and press enter. At the bottom of the results you will find a line saying "SERVER:" (see below). The IP address following (underlined below) is your Primary DNS server, so enter that into your Xbox. Don't enter anything under Secondary DNS server.

In Network Settings on the Dashboard, select "Test Xbox Live Connection". You should now be able to connect to Xbox Live.