It seems to be, according to comments recently made by Google’s CEO, Eric Schmidt:
Web 3.0 will be "applications that are pieced together" - with the characteristics that the apps are relatively small, the data is in the cloud, the apps can run on any device (PC or mobile), the apps are very fast and very customizable, and are distributed virally (social networks, email, etc).
Small apps? Check. Data in the cloud? Check. Customizability? Check. Well, Popfly’s getting there at least.
Props to ReadWriteWeb for the original video, quote and analysis.