iCloud and Document & Data Syncing
iCloud does not backup all of your data. It's more a replacement of iWork.com sharing in previous (or current?) versions of iWork. It only works with applications that specifically support iCloud:

If you have the same iCloud-enabled apps on more than one device, iCloud automatically keeps your documents up to date across all your devices. So you don’t have to.

If you visit http://icloud.com and click the ? link, you'll be sent to the help site, which states:

Documents in the Cloud overview
iCloud keeps your documents up to date across all your iOS devices and on icloud.com. Every time you edit a Keynote, Pages, or Numbers document on your iOS device, your changes are automatically available on your other iOS devices set up with iCloud and on the web at icloud.com.

This also explains why the iCloud site does not show a generic documents section in the overview, only iWork. Of course, developers are free to support iCloud in their own applications.

The supported file types are listed here:

You can also drag and drop a new iWork ‘09 (Keynote, Pages, and Numbers) or Microsoft Office (PowerPoint, Word, and Excel) document from your computer to icloud.com, and then access the document from Keynote, Pages, or Numbers on your iOS device set up with iCloud. Supported file types include Keynote ‘09, Pages ‘09, Numbers ‘09, PowerPoint, Word, Excel, .txt, and .csv.