I've written a patch which provides a new kernel boot param "i915.enable_backlight=0".
i915.enable_backlight=0 can be used to disable i915 backlight control
and the /sys/class/backlight/intel_backlight interface -- useful for
systems where intel_backight conflicts with BIOS backlight control.
An Ubuntu Precise PPA kernel with the patch applied is here: https://launchpad.net/~kamalmostafa/+archive/i915-enable-backlight
Mario/others, please apply the PPA kernel, then boot with "i915.enable_backlight=0". This should resolve the problem (it does on my XPS13). Please confirm.