Agree this is a bug, the problem is exactly the same on my IBM T60 2623-D6U, Ubuntu 12.04. It is possible to have "single step" brightness using 1) kernel parameter acpi_backlight=vendor and 2) thinkpad_acpi module parameter brightness_enable=0. But in this case OSD brighness change notification desappears.
Agree this is a bug, the problem is exactly the same on my IBM T60 2623-D6U, Ubuntu 12.04. It is possible to have "single step" brightness using 1) kernel parameter acpi_backlight= vendor and 2) thinkpad_acpi module parameter brightness_ enable= 0. But in this case OSD brighness change notification desappears.