Apple has issued an update designed to address the lingering problem of <A HREF="http://news.zdnet.com/2100-1040_22-6130183.html?tag=nl.r23660">unexpected shutdowns of its MacBook notebooks.</A> The MacBook SMC Firmware Update 1.1, released for download Thursday, tackles a problem that has plagued the notebooks for the past four months. Apple advises people to install the software on all MacBook systems, including those that have received warranty repair.