As we mentioned in our Blackra1n/Blacksn0w Jailbreak/Unlock How to Guide, GeoHot made Blacksn0w available via Cydia.
This option is great for the people who have jailrboken their phones but not unlocked or could not unlock it because of 05.11.07 baseband upgrade.
The Blacksn0w installation process is vey simple. Steps after the fold.
1. Open Cydia and install new source by going to Manage menu.
The source URL for the blacksn0w is http://blackra1n.com
2. After blackra1n source install, you will have access to Blacksn0w app.
Click Install Button
3. Confirm installation
4. Wait for installation process to finish
5. After installation make sure to reboot your iPhone in order for unlock to take effect.
Note: if you are on 3.1.2 and your phone is unlocked by Ultrasn0w, do not create trouble for yourself. Enjoy your current unlock, and do not bother with this method.
But you like to tinker, good luck.