Can the iPhone 3G / original iPhone (2G) support multitasking?
Not officially. Although iPhone OS 4 supports multitasking, Apple is limiting the feature to the latest-generation hardware. This means that the original iPhone and iPhone 3G are left out due to hardware limitations and performance concerns.
One intrepid developer has discovered that a single preference file turns multitasking on or off. If your iPhone is jailbroken you can edit the N82AP.plist file. Set multitasking=TRUE to switch on the feature. Applications for multitasking such as multifl0w and ProSwitcher have been available on Cydia for some time.
Unlike these jailbreak apps, Apple's iPhone OS 4 makes available 7 specific background services to make sure apps don't use too many resources when running in the background.