this link seems to imply the default database for a user might be null?
http://www.sqlservercentral.com/Forums/Topic1259661-146-1.aspx
do either of these return any data?
SELECT *
FROM sys.server_principals
WHERE default_database_name IS NULL
AND type_desc IN( 'SQL_LOGIN', 'WINDOWS_LOGIN','WINDOWS_GROUP' )
SELECT *
FROM sys.server_principals
WHERE default_database_name NOT IN(SELECT name
FROM sys.databases)
Lowell