Download Redsn0w 0.9.6 Beta 5 For Windows, Mac OS X by gsmiphone

As you are already aware, the iPhone Dev Team have followed an unusual method to unlock iPhone 3GS, 3G on baseband 05.14.02 or higher on iOS 4.2.1 and iOS 4.1. They have initially released PwnageTool 4.1.3 Unlock Edition that lets you upgrade to baseband 06.15.00 (of iPad 3.2.2 FW) for which they have a working exploit.
Ultrasn0w 1.2, the latest version, will let you unlock your iPhone 3GS or 3G on 06.15.00. Which means by upgrading your baseband from 05.14.02 to 06.15.00 you can essentially unlock your iPhone 3GS or 3G.
Now Redsn0w 0.9.6 Beta 5 has been released for both Windows and Mac OS X with an update to jailbreak iPhone 3GS and iPhone 3G while updating to baseband 06.15.00 that is required for Ultrsn0w 1.2 to work.
When you run Redsn0w 0.9.6 Beta 5, select corresponding IPSW (iOS 4.2.1 or iOS 4.1) for your iPhone 3GS or 3G and proceed you will be given an option to ‘Install iPad baseband’. Selecting this will update your iPhone’s bb to 06.15.00 which is currently the only way to unlock 05.14.02 on iPhone 3GS and 3G.

Redsn0w Crashing And Other Problems

Make sure that you place the files you need during this process on your desktop.
Note Windows Vista and Windows 7 users
Run Redsn0w in administrator mode (right click and select ‘Run as administrator’) in Windows.
You may have to use XP Compatibility mode if you encounter problems with Redsn0w 0.9.6 Beta 5.

The new redsn0w 0.9.6beta5 is out.  It gives both Windows and OSX users the ability to flash the iPad 06.15 baseband on iPhone3G or iPhone3GS.  It fetches the baseband files directly from Apple for now (the only IPSW you ever point it at is the stock IPSW for the FW on your iPhone right now).  There may be a long delay while it’s doing this (their servers are getting pounded right now).

Note Windows Vista and Windows 7 users: You may have to use XP Compatibility mode if you encounter problems with Redsn0w 0.9.6 Beta 5.
Download Redsn0w 0.9.6 Beta 5 for Windows

Download Redsn0w 0.9.6 Beta 5 for Mac OS X

How To Update iPhone 3GS, 3G Baseband to 06.15.00 Using Redsn0w 0.9.6 Beta 5

Here is a guide to update iPhone 3GS, 3G on iOS 4.2.1/iOS 4.1 to baseband 06.15.0o using Redsn0w 0.9.6b5.

Unlock iPhone 3GS, 3G

After updating baseband using Redsn0w, follow this guide to unlock with Ultrasn0w 1.2.