The iPhone Software.com

Once you do just that, hackintOsh Member sparx writes that you should:

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 ips.
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)
9. Activate the phone
10. It will show the SIM error but you should be able to get back in.

Additional stuff for code jocks here.