Well, the "what" sounds like the buffer of the desktop image is getting wiped somehow. That of course doesn't explain the far more important why, nor what to do to fix it.
I've occasionally seen that happen to a small area of the screen due to a badly misbehaving app, but rarely and never on that scale, so I'm a little at a loss as to what might be causing that.
Are you using any system hacks? What about any apps that might be doing something major with the desktop (like a desktop screensaver app, for example)?
Also, so it's clear what the issue is, do your desktop files show up ok and just the background goes white, or is EVERYTHING going white, icons and other windows included?