Safe Boot would let me proceed a couple of minutes beyond login, before the mac would shut down without any error message. The 10.6.8 Combo Updater also installs the newer mach_kernel at Root level, and again the MBP would shut down during boot, before the login screen. Restoring the older mach_kernel (March 6 2011) from back-up made everything normal again. An updated version of the mach_kernel file (creation date: June 8, 2011) made my MBP shutdown prematurely after applying the MBP Software Update ver1,4.
