To Jailbreak iOS 4.0 on an iPhone 3G (with multitasking support) do this: (also works on iPod touch 1st and 2nd generation)
1) Make sure you have iTunes 9.2 downloaded and installed.
2) Download the iOS 4 for iPhone 3G IPSW file from
here
3) update your iPhone 3G to iOS 4 by opening iTunes, holding shift and pressing
update then pointing towards the IPSW file you just downloaded.
4) Download redsn0w 0.9.5b5-3 for windows
here, or for Mac
here
5) Start up redsn0w and aim it at the IPSW file you downloaded and used to update on #3.
6) Now hit
install cydia and click on
next
7) Now before continuing, turn
OFF your device and make sure it is plugged into your computer via USB cable (it tells you on screen to do this too), then hit next
Follow the on-screen instructions on going into recovery, and wait while redsn0w jailbreaks your device for you. After-wards you will see you now have a Jailbroken iPhone.
note: if you open Cydia and it is just a white screen, go to
Sources >
edit >
add and add the repo
http://apt.saurik.com/cydia-3.7 then restart cydia and you should be good to go. For unlocking on any baseband see below.