it's a hype created by Tim O'Reilly, alot of theories floating around but some people think that it web 2.0 was created so that some people could capitalise on 'hyping' up a new wave of web design/development technologies, which so happens to be stuff that people have already been using for ages!
its been misinterpreted quite a bit too... refelective logos for example..
on the plus side it did make alot of designers/developers realise they needed to catch up with what the trends were and you see fewer websites with tables and alot more websites utilising cool stuff like ajax and some thought towards accessibility etc..