Your Mac may start up to a grey screen after upgrading from Mac OS X v10.5 to Mac OS X v10.6 Snow Leopard. This may occur if a previous Mac OS X v10.5 software update had been downloaded using Software Update, but not completely installed, before installing Snow Leopard.
If you have already upgraded to Mac OS X v10.6 and are experiencing this issue, reinstall Mac OS X v10.6.
If you haven't yet upgraded from Mac OS X v10.5, finish any incomplete software update installations by restarting your Mac. Once your Mac has restarted you can proceed with upgrading to Mac OS X v10.6 Snow Leopard.