What happens when you run with this diff?
Index: acpivout.c
===================================================================
RCS file: /cvs/src/sys/dev/acpi/acpivout.c,v
retrieving revision 1.9
diff -u -p -r1.9 acpivout.c
--- acpivout.c 23 May 2011 11:58:03 -0000 1.9
+++ acpivout.c 21 Oct 2011 09:44:17 -0000
@@ -115,6 +115,7 @@ acpivout_attach(struct device *parent, s
ws_get_param = acpivout_get_param;
ws_set_param = acpivout_set_param;
+ acpivout_set_brightness(sc, 0x0f);
acpivout_get_bcl(sc);
}