The death of email has been predicted before in 2002, 2004, and now again in 2007. This time it may be for real, at least in regards to personal email. This latest article from Slate outlines all of the reasons and none of them can be refuted. Email has given way to more direct communication methods, such as instant messaging, text messages and social networking sites (facebook, myspace) and tools (twitter).
Business email will trudge on though. Since this kind of email acts as a documentation trail, businesses and business processes will loathe to give it up. I, myself, use my gmail account as my poor mans CRM tool. Email is well integrated into technical infrastructure of most businesses, it's not going anywhere and its cost to maintain is dropping. There is a chance that Microsoft Sharepoint, wiki's or RSS feeds could supplant these but it will be a while.
Personal email is dead. It's been replaced by something faster and easier. In some ways, it was inevitable. People do not have the time to craft long missives to friends or family any more. Most people are operating in the "now". Many people are armed with their cellphone where ever they go. A few of those people have internet access too, right in their pocket. If they want to communicate, they do not want to fire up the computer (or find one in a public place), nor do they want to hope that the person on the receiving end is willing to do the same.
So yes, email is dead but not in the enterprise.