Is there a way to show the release dates of System software updates?
If you are running a software update service on OS X Server, you can see when Apple released each update (and also when the server actually downloaded the file). This is also nice as then you can then only mark an update as viable to your machines once you have had a chance to review things.

Also, for a more lightweight solution, you can look at the downloads web site to determine when a particular package was released.