You could see the writing on the wall when Apple announced iPhone OS 4. Apple said most of the features will run on the iPhone 3G, and all would run on the iPhone 3GS. At no point did they mention the first generation iPhone. Steve Jobs has now confirmed what many assumed.
MacStories has published an email a reader sent to Jobs asking if Apple will supply updates to the first gen iPhone in the future. Steve's answer? "Sorry, no." So unless Apple releases another minor point upgrade to iPhone OS 3 before OS 4's release, original iPhone owners can expect to be running iPhone OS 3.1.3 for as long as they keep the device.