This backdoor uses a security exploit in the activation screen.
How to upgrade to iOS 5 for iPhone 4,3GS ,iPod touch 4G, 3G and iPad 1, 2:Step 1: Download the iOS 5 IPSW file for iPhone 4 and iTunes 10.5 - Click Here.
Step 2: Install iTunes from the downloaded DMG file.
Step 3: Update your phone with itunes by connecting it and clicking on the "Check for Update" button while holding down the Option key (Shift on a PC). Select the iOS5 IPSW you just downloaded and confirm.
How to Activate iOS 5 without developer accountStep 1: Triple click the home button to activate Voice Over
Step 2: Triple click the home button and Emergency Call will appear.
Step 3: Click on Emergency Call and, while it's switching, swipe with your three fingers down.
Step 4: The Notification Center will appear!
Step 5: Click on the Weather widget and wait for the weather app to load.
Step 6: Click on the home button to exit to the iPhone's springboard.
There you go! Your iPhone is now running iOS 5.