Are you still having this problem? I seem to have solve my similar issue by resetting the screen background settings and desktops.
Before I started to see the "whitescreen" I'd copied over some default backgrounds from OSX 10.6.8 and set them as a desktop background. I'd had to individually set the same image to 3 desktops. I don't recall if I'd left any applications running when the MBP went to sleep and logged me out.
Assuming the "whitescreen" is some form of memory leak/corruption; I deleted the extra desktops and reset the background to a default OSX 10.8.2 solid color. Did a full power down of the machine and once restarted, let it go to sleep and log me out. Woke it up; no whitescreen: added a default Lion image, let it sleep. I repeated this process loading a copied 10.6.8 image; and then made three desktops, changed, added images to each desktop. So far, so good. Since I was "impatient", I'd cranked the sleep settings down to 1min so I don't know yet if there is a relationship between the whitescreen and sleep or log out delay time.
Hope this helps or is even relevant.