Xcode 4.2 install for Snow Leopard fails with an OpenGLApps.pkg
Dave, your post and subsequent answer were just what I needed, and very refreshing indeed.  I was getting the same error and log output, trying to install Xcode 3.2.6 with iOS 4.3 on OS 10.6.8 (the untrusted package in this case was iPhoneSDKTools.pkg).  I had tried a bunch of other things, to no avail.

When I followed your suggestion and checked for updates, it listed one for Safari (including some security-related fixes) plus another for the Application Installer (or something similar) - the description for that mentioned something about fixing problems installing certain apps on 10.6.8!  After doing those updates the Xcode install finally worked.  I'm pretty sure it was the Application Installer.  I didn't have to download a fresh Xcode installer, I just used the one I'd been trying to use all along.

Other posts I'd found recommended reinstalling Snow Leopard, which seemed a bit extreme and didn't answer why it was happening.  I think you've hit on the real issue and a nice straightforward easy solution.  Thanks.  I can now abandon my plans to leap into the Abyss.