I upgraded Debian Squeeze to Wheezy, but when Wheezy boots, my laptop, an Acer Aspire 7715Z, is refusing to switch on the LCD's backlight! I verified that the OS is booting by trying to log in as user and as root and by running an audio file and confirmed that in fact it is booting.

Wheeze lights the LCD's backlight if I use Squeeze's kernel linux-image-2.6.32-5-686.

Des anyone know of a patch or workaround?

Try disabling KMS.

KernelModesetting - Debian Wiki - http://wiki.debian.org/KernelModesetting#Disabling_KMS

I read that page already and spent several hours trying to figure out what I should do. I also edited the /etc/modprobe.d/i915-kms.conf file to disable kms like this:

options i915 modeset=0

The result was an extremely low resolution that makes my laptop screen unsuitable for modern application interfaces. If there is 3D acceleration I may be able to fiddle with Xorg.conf to get more resolution, but I don't know, and it is against the way Xorg is supposed to work.

If you have more ideas, I would be grateful if you post them.