I had a T23 that experienced exactly the same problem. In my case, the culprit was the little recessed switch that gets depressed when the lid goes down. One time when opening the lid, the switch came up off-axis. It got stuck there, with the switch imperceptibly still depressed. No amount of opening/closing would make the backlight come back to life. It wasn't obvious, but IBM's tech support people (unbelievably) diagnosed this over the phone. At their suggestion, I poked around the switch with a pen, and it finally popped up the rest of the way.

I have no idea how likely that is, but you might try this before you crack that case open.

fiat_owner