I have to agree with Alex's tongue-in-cheek definition. I have worked in interactive agencies and we would roll our eyes when the client would ask for something "Web 2.0." It came to be a bit of shorthand for "something cool like Flickr/Google Maps/Facebook." We eventually just learned it to mean "ajax." I did see a presentation recently by Jakob Nielsen at the IA summit in Vegas last month. He argues that Web 2.0 is characterized by social interaction but also through open APIs. He gave us the example of Twitter Vision, which is a mashup of google maps and twitter. Check it out if you have a voyeuristic streak and time to kill. Those in the industry these days are thinking Web 2.0 is already dead. They're thinking Web 3.0 is being built -- and I have to agree -- using things like open APIs in the mobile channel.