Blacksn0w RC2 to unlock iPhone iOS 4.0 has been released into Cydia. Blacksn0w RC2 is based on original BlackSn0w and binaries for baseband 05.11.07 released by MSFTGuy to unlock iOS 4.0 GM (aka iPhone OS 4.0) Baseband 05.11.07 for iPhone 3GS. BlackSn0w RC2 is not released by GeoHot. The guy behind Push fix 2.0 has repackaged it for you.
Blacksn0w RC2 Instructions From iPhoneHeat
Description from Cydia..
Use on Modem Firmware 05.11.07 ONLY!
V1.2: Support for 4.0 GM. Backwards compatible to 3.1.2/3 as well. V1.1: MyWi tethering fix
Geohot’s unlock for 3.1.2/3.1.3/4.0GM on the iPhone 3GS (and probably 3G). Please donate at http://blackra1n.com/ if you enjoy.
With 3.1.3 & 4.0 GM patch (when needed) by msft.guy. Packaged by Pushfix.info
This is an unofficial release but Geohot HAS given me permission to redistribute blacksn0w. All known issues with WiFi loss and boot loops seem to be fixed. If it doesn’t work for some reason, you should still get the SpringBoard. This allows you to uninstall if necessary. It is a little slower to boot because of the increased safety of the release. A 3GS boots to cell connection established in about 90 seconds in iOS 4 now (50% slower than 3.1.X).
Improvements may be made in the future.