If you are using Forms Based Authentication for you SharePoint site you might encounter the site displaying a user name such as "i:0#.f|sql-membershipprovider|username" instead of just "username".
You can modify the tp_Title field in your SQL Server UserInfo table directly, but at times the seems to be reverted back to its original value. There is a solution to this problem, however it is unsupported of course, as you are modifying a stored procedure in the SharePoint database directly.
In your SharePoint database located the proc_UpdateUserInfoInTableFromRowUpdater stored procedure.
Under the declaration section of the @OldIsActive bit field add the following code:
IF (SUBSTRING(@Title, 0,3)='i:0')
DECLARE @LastIndex int
SET @LastIndex = (SELECT CASE CHARINDEX('|', @Title)
WHEN 0 THEN -1 ELSE LEN(@Title) - CHARINDEX('|', REVERSE(@Title))+1 END)
SET @Title = SUBSTRING(@Title, @LastIndex, LEN(@Title)-@LastIndex)
You might have to modify the first line of code based on the way the tp_Title is displayed in your database.