Apple has not been shy in stating that the original iPhone 3G will not support multitasking but iPhone hacker cdevwill has proved them wrong - sort of. After the break you can see a YouTube video of an iPhone 3G running iPhone OS 4 jailbroken with functional multitasking.
While this all seems like a bed of roses stop and think about it for a minute: the only apps someone could test currently are those built in the iPhone OS 4 software (many of which have always multitasked) - no 3rd party apps. You'll notice in the video the amount of time it takes to open the clock application. Now imagine trying to play a game like Plants vs. Zombies and checking your Twitter all while streaming Pandora.
So while multitasking on an iPhone 3G appears to work, don't expect a very smooth experience. This is more than likely the reason Apple claims it will not work (even if they'd rather you upgraded your hardware).
If you want to learn more about jailbreaking in general stop by our forum dedicated to the topic and be sure to check out the video after the break!