I agree with Vincentt. My data is already in the cloud. I long ago let go of my physical storage devices, more out of necessity than anything (I bike tour, and move a lot). My mail is on gmail, my chat logs are on meebo, my files are on my iDisk, photos on Flickr etc etc. Web sites are becoming thin clients. More and more people will use these services as they mature and become more powerful. But that’s all Web 2.0.
Web 3.0 is what happens when we figure out how to take our data, our friends’ data, and all the rest of the data from well-known experts and complete strangers, and make something really awesome out of it.
Some examples of this include Mint.com which draws all your data from various financial institutions into one Quicken style application. Knowing everything it does about people’s spending trends, it can then tell you if you spend more or less than other people in your city on everything from gas, to dining. This is just one example that targets a very specific niche; personal financial data.