A quote taken from a CNET article on tips and tricks of Google Docs:

5. There are many copies. And they have a plan. The first thought in most people's minds when they're working with online apps is, "Where is this file being stored, and what if something bad happens?" Any document or spreadsheet created on the service is constantly being backed up in several places at once. Google uses the same file system for all of their Web apps, called GFS (Google File System), that's been designed so even if the server in which your file is hosted bursts into flames, the system will automatically switch over to the backup copy. The team says if this were to happen, users wouldn't even notice.

Yup, Google is smarter than me, and has stronger resources than I do. That's why I trust storing my stuff in the cloud more than I trust storing my stuff on my own computer.