A hack called Whited00r allows iOS 5 features to be installed on the original iPhone and iPhone 3G devices, as well as the early iPod touch models.
Whited00r will automatically jailbreak your device, and the latest version of Cydia, 1.1.3, will be installed on it. The biggest advantage of the tool, is that you will get true multitasking, and apps will keep in running state in the background.
It's as simple as running a manual IPSW install via iTunes before Whited00r takes over and installs a custom iOS firmware package. Apparently you'll be running a heavily modified iOS 3.1.3 IPSW, which is required on older iOS devices. As such, this version of iOS comes with a custom iCloud setup, and also doesn't include the new Notification Center included in iOS 5.