To enable iOS 5 beta 2, you must install iOS 5 beta 2, iTunes 10.5 beta and a Wi-Fi connection. Then follow the steps below as on iDB:
1: Install iOS 5 beta 2 on your iOS device.Here's a video in action:
2: Install iTunes 10.5 beta on your computer.
3: Plug your device into your computer and open iTunes.
4: In iTunes under the summary tab for your device, enable “Sync over Wi-Fi connection” and sync your device via the usb connection.
5: Untether your device from USB, and your device will still be recognized by iTunes!
6: Drag and drop files from iTunes to your device, and wipe the tears from your eyes as you watch Wi-Fi sync work for the first time.
[via iDB]