Hi - I have the MacBook Pro 13" (5,5) and I experienced this issue. Terrifying! For a second I thought the LED died. Seeing the the dark screen & "breathing lamp" twigged me to the fact that Sleep was now a default. I hit the Space Bar and the mac woke a bit but it's like hitting Snooze - it went right back under.
Next I Powered-On and decided to boot to my Bootcamp install of Windows. I held down alt/option. While holding down the alt/option the screen came on and I let it sit for 5 minutes before selecting to boot the Vista side. I was happy to see the screen hardware functional for 5 minutes - since twice now under OSX AND under Windows, the screen went dark after about 2 minutes. Vista booted, screen was on until the final moment (and Vista booted perfectly well) but the display again goes dark! The keyboard lit up (dark room) under Vista. I pressed the Brightness keys - nothing (but I could make the keyboard go bright and dark). Under Vista IT DID NOT SLEEP! I hit Power and it shut down properly.
I called Apple. It turns out that MacBook Pros have an issue with the NVIDEA chips (going back to 2008 - current) where the chip blacks out the screen at certain temperatures / in my case when the machine is cold. Apple will replace your NVIDEA chip if they have a look at your MacBook and prove the issue. No charge (except inconvenience and lost time).
http://support.apple.com/kb/ts2377
http://discussions.info.apple.com/thread.jspa?threadID=1727530&tstart=15&start=0