An updated sign-in client for Office for Mac 2011 is due for release at the end of March. The downside for those running older versions of Office for Mac or OS X is that the new sign-in client won’t work for you. You can read more about it here: http://blogs.technet.com/b/msonline/archive/2011/03/02/new-sign-in-application-supports-office-for-mac-2011.aspx
Loryan Strant, Microsoft 365 MVP
Consultant, writer and speaker