Try with objectClass=''User''. From this example:


SELECT * FROM OpenQuery(ADSI, 'SELECT title, displayName, sAMAccountName, givenName, telephoneNumber, facsimileTelephoneNumber, sn FROM ''LDAP://DC=whaever,DC=domain,DC=org'' where objectClass = ''User''')

Found here: http://codebetter.com/blogs/brendan....2/19/4746.aspx