**NOTE: It seems like most are able to capture both their iBEC and iBSS files in DFU mode. To try it just in DFU mode skip steps 4 through 6.
Step One
Press the Windows key + R to bring up the run dialog. In the entryfield type %temp% then click OK.
This will bring up the windows temp folder for your user account. Make sure you order the folder by date modified with the most recent at the top. Scroll down to the top of the folder list.
You must be able to view hidden files and folders. In Vista you can do this by selecting Organize from the toolbar then choosing Folder and Search Options.
From the View tab of the window that appears choose Show hidden files and folders then click OK
Step Two
Create a new folder on your desktop called Pwnage and place the 3.0 firmware in the folder. You can download the firmware from here
Step Three
Launch iTunes from your Programs menu.
Step Four
Make sure your iPhone 3GS is connected to the computer then select from the list of devices on the left. Hold down the Shift key and click the Restore button
Select the iPhone2,1_3.0_7A341_Restore.ipsw file from the Pwnage folder on your desktop and click the Open button.
Your iPhone will now begin restoring to the 3.0 firmware. You will be warned that this will erase your phone. Click the Restore button to continue.
Step Five
As the restore is progressing watch the %TEMP% folder we have open. A new folder will appear with a name similar to Per518D.tmp. When the folder does appear, select it, then press Control+c to copy it to the clipboard.
Move to the second Finder window that is showing the Pwnage folder and press Control+v to copy the file folder into the window.
**Remember you only have a limited time to do this as the folder will be removed very shortly. It seems like a new folder will pop into the %TEMP% directly three times during the DFU restore. You need to capture the first one that appears.
Step Six
Once the restore has completed iTunes will ask you to restore from backup or set up as a new phone. To keep your settings as before select Restore from the backup of: then click the Continue button
Step Seven
Once the backup has been restored we will need to do another iPhone restore from DFU mode to get our iBSS. To do this follow these instructions to put your iPhone into DFU mode.
iTunes will popup a message saying it has detected an iPhone in recovery mode.
Click okay to this message then Hold down the Shift key and click the Restore button
Select the iPhone2,1_3.0_7A341_Restore.ipsw file from the Pwnage folder on your desktop and click the Open button.
Your iPhone will now begin restoring to the 3.0 firmware. You will be warned that this will erase your phone. Click the Restore button to continue.
Step Eight
As the DFU mode restore is progressing watch the /tmp folder we have open in Finder. A new folder will appear with a name similar to PerD112.tmp. When the folder does appear, select it, then press Control+c to copy it to the clipboard.
Move to the second Finder window that is showing the Pwnage folder and press Control+v to copy the file folder into the window.
**Remember you only have a limited time to do this as the folder is removed shortly.
Step Nine
Once the DFU restore has completed iTunes will ask you to restore from backup or set up as a new phone. To keep your settings as before select Restore from the backup of: then click the Continue button to finalize the restore.
Step Ten
That's it. We now have captured our iBEC and iBSS. You can move these to a safe place to be used for a future jailbreak of your iPhone 3GS.
To confirm you have the files look in Pwnage/Per????.tmp/Firmware/dfu. You should see two files: iBEC.n88ap.RELEASE.dfu and iBSS.n88ap.RELEASE.dfu
