If you will jailbreak iOS 4 on iPhone 3G using Redsn0w 0.9.5, you can enable "Multitasking" option your iPhone 3G easily. But it will also upgrade your iPhone's baseband to 05.13.04. So i would advise you to jailbreak your iPhone using PwnageTool 4.0 or Sn0wbreeze 1.6. Jailbreaking iOS 4 on your iPhone with the said tools will preserve your baseband but don't offer you to enable "Multitasking" while jailbreaking your iDevice.
The following guide will explain how you can enable "Multitasking" on your iPhone 3G even if your jailbreaked your iPhone via PwnageTool or Sn0wbreeze.
Activate "Multitasking" on iPhone 3G Running iOS 4
- First step is to install iFile on your iPhone / iPod Touch. You can find step-by-step guide here for iFile installation on iOS 4.0
- Open "iFile" and and go to "root" directory. Tab on top left corner of your screen, it will take you root (/) directory.
- Go to /System/Library/CoreServices/SpringBoard.app and look for N82AP.plist file.
- Once you will open N82AP.plist file, you will see the coding regarding your iPhone functions, you will need to edit the file and add "Multitasking" function to it and set it to "TRUE".
- Tab on "Edit" on top left corner of your screen.
- To enable Multitasking add following strings to N82AP.plist file.
- One you are done with adding these strings, just tab on "Save" and "Done" and close the iFile app.
- Restart or respiring your iPhone 3G and enjoy multitasking on it.