tl;dr iOS5 temporarily bricked my iPhone4, but iTunes restored it with ⌥ + restore fixed it.
While upgrading from iOS 5 beta2 to beta4, my iPhone4 stopped responding, and the screen wouldn't even come up. Home+Power didn't do anything, and XCode wouldn't even recognize the device. Luckily, iTunes was still picking up the device - but when I try to restore the phone, it would fail with a vague error message (error number 1600 or something). After setting up a Genius Bar appointment and googling around for about 30 minutes, I found out that this error is because I am trying to install iOS 4 on a device that has iOS 5. Then I read shift + restore (actually, it was Option + restore for me) allows you to pick the ipsw file.