This is not new technology. Clarity and Zoe have been doing this for email in a cross platform styleee (unlike Google) for years. What is new, is that Google has the resources to make this work with most everything not just email. And Google can offer an API to enable the indexing of funky files so it works with literally everything. Finally and most importantly, Google can market it!

From :O'Reilly Network: Google Your Desktop.:

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Google your desktop and the rest of your file system, mailbox, and instant messenger conversations--even your browser cache. Not content just helping you find what you need on the Internet, Google takes on that teetering pile on your desktop--your computer's desktop, that is. The Google Desktop is your own private little Google server. It sits in the background, slogging through your files and folders, indexing your incoming and outgoing email messages, listening in on your instant messenger chats, and browsing the Web right along with you. Just about anything you see and summarily forget, the Google Desktop sees and memorizes for you. And it operates in real time.

UNQUOTE