Macbook Pro screen goes blank when disconnecting Magsafe adapter

Twice in the last couple of days (since installing 10.5.6), my Early 2008 MacBook Pro has gone dark when I've unplugged the power adapter while it's been charging and powered-on. It keeps running fine, but the backlight can't be turned back on, even when I try to change the screen brightness with the keyboard. I can dimly see the brightness indicator change on the screen, confirming that it's hearing my keypresses.

The only way I can restore the display is to restart the machine. Has this happened to anyone else? Only major change was the install of 10.5.6 update a couple of weeks ago. I have had the dreaded NVidia distorted video problem about 6 months ago, but that's not happened since. Machine temperature doesn't seem excessive--CPU is at about 130 F when charging.

Thanks in advance