Not sure what you guys mean with integrating with Google Apps, but I am in a similar situation and have written a plugin to (ab)use Google Apps to authenticate my users. This plugin allows the users in our Google Apps domain to login in to our (externally hosted) wordpress installation (which is a closed environment for authenticated users only).
It's a kind of a hack, but instead of logging into wordpress separately, it redirects the user to Google Apps to request the user for authorization to access the user's calendar. If tbe user accepts, Google will re-direct the user back to my wordpress installation and my plugin will find out using the token passed by Google who granted authorization. The user is then authenticated without having to sign-in again and we only have have to add new users centrally with Google (the plugin will automatically add new users when needed).
It's not fully integrated with Google Apps, but the best I could think of now until Google Apps authentication is open for 3rd party apps. So far this works good enoug and I am in the process of publishing this plugin (just need to add documentation and some additional checks here and there).
Let me know if anyone is interested to test this already...