It might be a problem with the bcm43xx driver. Indeed, I experienced several lockups with debian ppc on my ibook. The last message was exactly the same as yours, where bcm43xx restarts the controller.
Oct 29 10:28:08 protheus2 kernel: bcm43xx: Radio turned off Oct 29 10:28:08 protheus2 kernel: bcm43xx: Radio turned off Oct 29 10:28:08 protheus2 kernel: bcm43xx: Controller restarted