Apple has said that the iPhone 3G would get some of the features of iPhone OS 4.0, but not all. One of the main features which the iPhone 3G wouldn’t get would be multi tasking, and the reason that Apple has given for this is due to hardware limitations.
iPhone Hacker cdewill has jailbroken the iPhone 3G and installed iPhone OS 4.0 and managed to get the multi taking feature running on the 3G with a simple modification to the N82AP.plist.
While multi tasking could be enabled on the iPhone 3G, it would be incredibly slow especially if you had a number of applications open at once. It stands to reason that a number of iPhone 3G owners will want this feature and will end up jailbreaking their iPhone’s in order to use it.