SOLVED (for my laptop at least - Acer Aspire 5740)
The trick was to add "acpi_osi=" (without the quotation marks) to the GRUB parameters. Setting the parameter to an empty string means that no OS is reported to the BIOS. By default acpi_osi is set to Windows NT. I had previously tried "acpi_osi=Linux" but it didn't work, apparently it had to be an empty string.
SOLVED (for my laptop at least - Acer Aspire 5740)
The trick was to add "acpi_osi=" (without the quotation marks) to the GRUB parameters. Setting the parameter to an empty string means that no OS is reported to the BIOS. By default acpi_osi is set to Windows NT. I had previously tried "acpi_osi=Linux" but it didn't work, apparently it had to be an empty string.
The brightness keys work now :-)