A thread at the Hackint0sh forum bubbles with glee. To quote forum user Sparx's refinement of a technique originally reported by Kmac1985:
"These instructions currently only work with iNdependence on the Mac. It
will unbrick you iPhone -but- will not restore cellular service, only
wifi.
1) Make sure you have a copy of the 1.0.2 firmware handy in ipsw and decrypted form
2) Reboot iPhone holding top (power) and home buttons *BUT* release the
top button 10 seconds into it (right after the screen goes dark)
3) The iPhone screen will appear to be off, but start iTunes (latest version worked for me on the Mac)
4) Option+Click restore and select the 1.0.2 firmware ipsw
5) The phone will restart and error out at the end with an unknown error 1013, this is expected
6) Restart iTunes and let it get to the screen that says you have an invalid SIM
7) Launch the latest iNDpendence
8) Jailbreak the phone using a decrypted 1.0.2 (it will flash all sorts of error, but stick with it)
clipped from: blog.wired.com 