Starting from Linux version 2.6.23, kernel passes OSI string "!Linux" to ACPI (instead of "Linux"). There is a number of reasons for that (see details in drivers/acpi/osl.c in the kernel source). But IBM/Lenovo has a consistently good record of supporting Linux, so this trick is not necessary for Thinkpads. Moreover, OSI string "Linux" is needed to make the Mute button work. For Thinkpads R61, T61 and X61, there are already hooks for that in drivers/acpi/blacklist.c. For most other models, you need to pass the string acpi_osi="Linux" to the kernel as a boot parameter.