I had the same problem on my Acer 4736 laptop having GM45 chipset, 4500MHD. I tried out adding 'nomodeset acpi_backlight=vendor' to the GRUB_CMDLINE_LINUX_DEFAULT line. After rebooting the brightness keys started to work, though the keys were revered. Unfortunately the brightness resets after subsequent reboots and some times stops working.
Anyone has any idea what could be the problem? Thanks in advance