This tiny manual comes without any warranty whatsoever! I only collected information from other sources, which seem to work for me, but not necessarily for anyone else. In fact you will even lose Apple’s warranty with a jailbroken phone. So use it at your own risk.

Here’s how it worked for me without any problems using MacOS:

Needless to say that you must take a backup with iTunes first!

Then, you need to download a firmware for OS 3.0 (not 3.0.1, even if you are currently running 3.0.1), parts of which will later be patched and then uploaded by the jailbreaking application. As 3.0.1 only adds SMS-related security fixes (they won’t be removed), this does apparently work, whereas using a 3.0.1 firmware does not work.

So, open a terminal window and download the firmware file  (not via Safari, because it unpacks the firmware bundle automatically, which then cannot be detected by the application):

curl -o iPhone2,1_3.0_7A341_Restore.ipsw
"http://appldnld.apple.com.edgesuite.net/content.info.apple.com/
     iPhone/061-6582.20090617.LlI87/iPhone2,1_3.0_7A341_Restore.ipsw"

(That goes all into one line, in particular the URL. Ran into display problems with this blog, sorry)

Secondly, you need the redsn0w torrent. If you haven’t got a bittorrent client yet, get the original here.

After you’ve downloaded redsn0w, simply run the application, and point it (“browse”) to the previously downloaded firmware file. The rest is absolutely self-explanatory. 3-5 Minutes later, your iPhone 3GS will be jailbroken, and you will be able to use the Cydia and Icy repositories with all their great (but not Apple-approved) iPhone applications.

When you open iTunes next time, you can easily verify that it still shows Software Version: 3.0.1

Have fun — at your own risk!

References:

• http://discuss.gdgt.com/apple/iphone/3gs/tips/iphone-3gs-background-apps-with-jailbreak/
• http://blog.iphone-dev.org/post/126908912/redsn0w-in-june