Apple uses digital certificates and code signing in various ways to help keep Macs secure. One common example is that apps sold through the Mac App Store are digitally signed, which allows an individual Mac to know that it’s getting the genuine article when a user launches the App Store app. It also allows a Mac to ensure that an application hasn’t been tampered with by a malicious user or a piece of malware each time that app is launched (Mountain Lion’s Gatekeeper feature will be based on the same technology).
The same process is used with Apple’s Software Update servers. Each update from Apple is digitally signed using a certificate that let’s each Mac know that they’re getting genuine updates from Apple.
Digital certificates are designed to expire periodically and tomorrow, March 23, 2012, the certificate associated with Apple’s Software Update functionality will be expiring. Apple already has a new certificate ready that won’t expire for seven more years (2019). The transition to the certificate will be transparent for almost all Mac users, but it may create problems with some OS X Server installations.