Google Apps single sign on can be achieved using the EasyConnect solution from SSO Easy.
One deployment option which is available through EasyConnect involves leveraging Active Directory login credentials, by enabling a password sync between AD and Google Apps. With this solution approach, Google Apps customers are able to enable web-based Google Apps SSO, as well as integrated SSO access to Google Apps from "alternate clients." including iPads, Mobile devices, IMAP / POP3 email clients, etc.
Free Trials of this solution approach are available at www.ssoeasy.com Free Trials are typically completed in less than 1 hour.