On a thread on freebsd-stable@, John Baldwin has diagnosed this as a conflict between the ACPI resource list and the BIOS.
http://www.mail-archive.com/freebsd-...msg107020.html
I can get the driver to work somewhat if the ACPI resource handling is disabled from loader.conf:
debug.acpi.disabled="sysres"
This works if the internal monitor is used; however, any external monitors are ignored.