Very related to this question: SSO, using Google Apps user database but I'm wondering how the user Nil started off with his OpenID script.
Could anyone give some background on this? I'm not very familiar with OpenID.
The Google Apps Marketplace Google Apps Platform Single Sign On overview provides all the information you need to get started. Since @Nils describes a Corporate Google Apps environment, it's very likely that this is where they went to start their implementation, since...
For in-house apps developed with the Google Apps extensions console, implementing Single Sign-On is a strongly recommended best practice.
You'll find background information and links to existing OAuth libraries in a variety of languages.
You should also look at the Google Identity Toolkit, which provides an API you can use to implement SSO for your web app, as well as a Javascript widget you can incorporate to make the task simple.