This blog post walks you through the features added to Update Rollup installation process starting with Update Rollup 1 for Exchange 2007 Service Pack 2, and what's happening behind the scenes.
"In addition to common system checks - such as disk space requirements and system integrity validation - we've added several Exchange specific pre-installation checks which're run prior to any modification of Exchange server: "First, a permissions check makes sure that you've logged onto domain rather than a local computer account. Next installer make sure that a .Net Framework hotfix is installed on computer. Next, installer determine if Certificate Revocation List checking is enabled, and if yes, whether we can reach CRL website. If CRL checking is enabled and CRL website is unavailable then NGen process can be dramatically more sluggish while it waits for CRL checking timeouts to occur. If this condition is detected, following warning dialog will be displayed." writes Microsoft.
More Info: Overview of Exchange 2007 SP2 Rollup Installation