With today's launch of Google Apps Premier Edition, we're proud to announce the availability of new administration APIs for fusing your business with Google Apps:
Single sign-on - Use this SAML-based interface to drive a consistent and unified login experience across all of your web applications.
Provisioning - Use this GData API to integrate with your existing user management system.
Don't forget, you can integrate with and extend many of the individual Google Apps services -- create gadgets for your start page, build custom visualizations on top of spreadsheets, synchronize calendars, archive your mail with a mail gateway, or build a video sharing tool on top of Google Talk.
Check out the Google Apps APIs page for more details!