iOS 5 is currently available to iOS developers only but one developer has find a workaround to make it installable on any device without actually needing a developer account.
Warning: We are not responsible for what you do to your iPhone. Once completed, you will not be able to connect to a carrier. Proceed at your own risk.
The method essentially exploits a vulnerability in the activation screen.
How to Install iOS 5 Beta
1. Google for iOS 5 beta ipsw for your iDevice and download it via torrents.
2. Update your iOS device using iTunes. click on the Check for Update button with the Option (Mac) or Shift (PC) key pressed. browse iOS 5 IPSW file.
3. Sit back while it updates. Wait till activation screen appears.
How to Activate iOS 5
1. Triple click the home button on your iPhone, this will activate the Voice Over.
2. Now again, triple click the home button and Emergency Call will appear.
3. Click on Emergency Call and, while it’s switching, swipe with your three fingers down.
4. The Notification Center will appear!
5. Click on the Weather widget. The Weather app will load.
6. Click on the home button to exit to the iPhone’s springboard.
That’s all. You would now have access to iOS5 homescreen bypassing the activation. What’s next? Jailbreak your iOS 5.