If you've been using Software Update (like I have, up 'til the other day), you've probably missed one of the new features of Leopard: Installer Certificates. Major updates from Apple now come with certificates of authenticity.
So, for instance, download the standalone Mac OS X 10.5.2 installer and launch the package in the Installer application, and you'll notice a small, new certificate badge in the upper right hand corner of the installer window.
Apple Installer: Certificate Badge
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Click the badge and you can take a look at the certificate that's attached to the installer, replete with details about said certificate under the "Details" disclosure triangle.
Apple Installer: Certificate Details
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It's another preemptive step in the right direction, security-wise. Always nice to see.