This just started happening to my 3GS recently. It is very much intermittent. Sometimes I turn it "off" with the top button, wait 10sec, and press either that button or the home button and it will show the normal unlock screen. Seems like 50% chance of good last week, a whole day of 100% good, and 10% today. The screen is not pure white: it appears off-white with a block of a pastel color. Thanks for the suggestion of turning off auto-lock; to reduce unintended actions, I am using it with the "status" (weather, stocks, etc) screen that appears after a full downward swipe. Obviously, this is only a temporary workaround. Intermittent problems can generate endless hypotheses. Since others encountered this about 2 years ago, I can conclude it has nothing to do with a recent update. And since the problem occurs when waking it up via the top button or the home button, the buttons are not to blame. It might be a corroded lcd connection that exhibits hysteresis. Perhaps a lengthy no-autolock On period would help it.
