Can't Login to SQL Server 2008 when running single user mode
This might be because you are attempting to connect using SSMS.  Inside SSMS, Object Explorer will open one connection to the database, while each subsequent query window will open another connection to the database.

What I have done in the past is open SSMS, do not connect to any database via Object Explorer, and open a Dedicated Administrator Connection (DAC) query windows by doing the following:


Click New Query
Type the following into the Server Name dialog box:  ADMIN:ServerName\InstanceName
From here, you can run any scripts needed against your database in Single User Mode.


This requires the SQL Server Browser service to be running.

The real sticking point in all of this is that Object Explorer will eat up that "single user".

Hope this helps,

Matt