iOS 5 beta 5has been released recently. It is available only for Apple registered developers. But you can try it too. Remember that you do it at your own risk. Remember your UDID must be registered at Apple in order to activate your device after install.
Developers can download it here or update via OTA (over the air). Prior OTA all content and settings on the device must be erased (Settings > General > Reset > Erase All Content and Settings). The delta update is just 128MB.