OS X Lion v10.7.2 and later
After updating to OS X Lion v10.7.2 or later, prepending the domain/workgroup name to the username is no longer necessary.
OS X Lion v10.7 and 10.7.1
If Lion Server is bound to Active Directory, a member of Open Directory, or is configured for a different Microsoft Windows domain than the client, the authentication may not work with the correct username and password.
To resolve this, specify the domain/workgroup of Lion Server with the username in the authentication dialog. For Active Directory, use the first part of the domain name; for example, for ads.example.com, put ADS\ in front of the username. For Open Directory, specify the first part of the DNS name of the server. If connecting to a server with DNS name lion.example.com, put LION\ in front of the username.