...If you're willing to jailbreak it, that is. Under normal conditions, the OS 4.0 update will not give the iPhone 3G the ability to multitask. Apple believes that the addition of multitasking will slow down the aging smartphone and shrink its battery life. Just because Apple doesn't want it done, though, doesn't mean it isn't possible.
BGR reports that iPhone hacker cdevwill was able to get the 3G multitasking, using the 4.0 jailbreak. The demo he gives is while using some fairly wimpy apps, and it's unlikely that a 3G will be able to handle any intensive processes. Still, multitasking is multitasking! Kudos to cdevwill for sticking one right in The Eye of Jobs.