I revised the instructions for the USB Method, hopefully its easier to understand:
Steps:
1. Download and extract the beta from:
http://www.multiupload.com/I2HU90MU5F
2. Plug in your USB memory stick into your Xbox
3. Goto My Xbox > System Settings > Memory > USB Storage Device > Configure Now
4. Wait while the Xbox reformats your memory stick and then plug it into your PC
5. Download USBXTAFGUI_v44 and run it.
http://www.mediafire.com/?cezh6acsioosp95
6. Goto File > Open First USB Device
7. It should load from your memory stick and under "File System" you should see "Cache Partition" and "Data Partition"
8. The folder structure for the memory stick needs to be the following. Create folders as needed (Note: thats 16 zeroes):
Data Partition > Content > 0000000000000000 > 4D5389B9 (drag extracted beta folder here)
9. Goto File > Close Current Drive
10. Unplug the memory stick from your PC and plug it back into your Xbox.
11. Goto My Xbox > Game Library > Game Demos & Trials and look for Gears of War 3 Beta
12. Load the launcher and it will prompt you to download a 500~ mb update.
13. You're done!
Note: This has been confirmed to work by a number of folks including myself. However, there are no guarantees as to whether this will get you banned or not. Proceed at your own risk!