I have an application with user authenticaton against database. The property I use is email:
[2013-10-21 23:49:19] security.DEBUG: Remember-me cookie detected.  
[2013-10-21 23:49:19] doctrine.DEBUG: SELECT t0.id AS id1, t0.username AS username2, t0.salt AS salt3, t0.password AS password4, t0.email AS email5, t0.is_active AS is_active6, t0.organisation AS organisation7 FROM User t0 WHERE t0.email = ? LIMIT 1 ["roger"] 
[2013-10-21 23:49:19] security.INFO: User for remember-me cookie not found.  
[2013-10-21 23:49:19] security.DEBUG: Clearing remember-me cookie "REMEMBERME"  
You can extend the default remember me service class and override the onLoginSuccess method so it uses the email instead of the username.