These are instructions on how to jailbreak your iPhone 3GS on the 5.0b4 firmware using RedSn0w for Mac. Since iOS 5.0 has not yet been released these are instructions for developers only. This procedure will cause you to upgrade your baseband potentially ruining your chances for an unlock, it will not hacktivate so you must have your UDID registered with a developer, and it is tethered.
Create a folder on your desktop called Pwnage
Download RedSn0w from here and place it in the Pwnage folder. Likewise, download the latest 5.0b4 firmware from the iOS Dev Center and place it in the same folder.
5.0b4 (3GS): iPhone2,1_5.0_9A5274d_Restore.ipsw
Extract the RedSn0w zip file by double clicking it.
Connect your iPhone to the computer and launch iTunes.
Select your iPhone from the list of devices on the left. Now hold down Option and click the Restore button. Restore is preferred as it won’t create any wasted space on your iPhone.
Navigate to the Pwnage folder on your desktop and select the 5.0 firmware ipsw. Click the Choose button to continue.
*Important: if you previously updated your iPhone 3GS to the 6.15 baseband you must launch RedSn0w and place your device into PwnedDFU before restoring to iOS 5.
Once iTunes has finished updating your iPhone to the desired firmware open the Pwnage folder on your desktop and launch the redsn0w application from the redsn0w folder we extracted earlier.
Once RedSn0w opens click the Browse button
Select the 5.0 firmware ipsw we placed in the Pwnage folder on our desktop then click Open.
Once the firmware has been verified click the Next button to continue.
RedSn0w will now prepare the jailbreak data
From this window you can select the jailbreak options you would like. Make sure Cydia is selected.
Click the Next button to continue
Please plug your iPhone into the computer and make sure its OFF then click the Next button
RedSn0w will now guide you through the steps to get into DFU mode. You can find more help with DFU mode here
Hold down both the Home button and the Power button for 10 seconds.
Release the Power button and continue holding the Home button until RedSn0w detects the device.
Your iPhone will now reboot
RedSn0w will then begin uploading the new RAM Disk and Kernel.
Once this is complete you will be notified that RedSn0w is done. Click the Finish button. When your iPhone finishes rebooting (5 minutes or so) it will be jailbroken with Cydia on the SpringBoard.
Since this is a tethered jailbreak you must use Redsn0w to help you boot back into your jailbroken device if its powered off for any reason. Simply connect your iPhone to the computer, launch RedSn0w, select Just boot tethered right now from the options window and click Next to have RedSn0w boot your device.