Slush's pool no more has user defined difficulty. For some time already, it sets the diff automatically. (But I can't say for what share frequency.)
[edit] Although, Slush hasn't updated the pool thread subject in this forum yet... :-)
He ended by doing it on Dec 9, after quite a long time since the actual change in September... :-)
He did so 5 days after my OP here and one day after OoC's including the change in his list. :-)