We were first alerted to this by iClarified reader Will Peterson who performed the update on two iPhone 5s. We have tested and confirmed it working on the iPhone 4S, a brand new iPhone 5, and an additional iPhone 5.
To perform the update you must get a hold of the iOS 7 ipsw, press and hold the Option key (Shift for Windows) and click Update in iTunes. Select the iOS 7 ipsw from the popup window to install.
It's unclear why this is working. Only iPhone devices registered by developers are supposed to be able to install beta firmware. Perhaps Apple took down this check since its servers are being flooded at the moment.
- Do not do this if you want to retain your jailbreak. You will not be able to get it back.
- Do not do this if you need hacktivation. You will not be able to hacktivate.
- Do not do this if you want to downgrade back to iOS 6. You will not be able to. Update: We tried to downgrade and it worked!