When you install SAM 4.5, you can choose (via a Custom Install) between installing a Fat version or a PowerPC (or 680X0) only version. I am not certain here, but it appears that an Easy Install installs the Fat version. If (for example) you installed the Fat version of 4.5 but download the PowerPC only version of the 4.5.1 Update, the Update Installer will not work. The quickest solution is to go back and install the PowerPC only version of 4.5; the Updater should then work. (Thanks, Steve Stitle and Dave Cuddy.)
Update: Some readers claim even this advice did not help them to get the Installer to work. I give up for now!