Can I upgrade to iPhone OS 4.0 on my original (2G) iPhone?
No. At this time reports have indicated that first generation iPhones (also known as 2G) will not support firmware version 4.0. iPhone 3G, iPhone 3GS and the fourth-generation iPhone hardware will all support the new operating system. This leaves the iPhone 2G stuck at iOS 3.1.3.
iPhone OS 4.0 has been described by some sources as making the iPhone act more like a full-fledged computer.