I have the same problem that these two discussions had..
http://community.office365.com/en-us/forums/160/t/60350.aspx
http://community.office365.com/en-us/forums/162/p/61963/233159.aspx
A few users are giving me the problem. Its nothing to do with the computer. On the same computer or any computer most users autodiscover with outlook 2007/2010 just fine, but two users simply will not. But I can log into the OWA with the bad users no problem. I think its something to do with deleting a user or changing their name and then creating a new user with the same name again, but I have no idea how to fix it.
You don't get past the authentication, the 2nd green checkmark doesn't come up. Sometimes you get all three checkmarks, but it keeps on asking for username and password. I have the username and password right because I can login via the OWA with that user!
Any help would be awesome,
Konrad