(IN() remove NULL rows only for tables from first argument (value which we looking for in IN() list) but not for tables from IN() list)
Also it will be better change Item::not_null_tables() to prohibit this optimisation by default for new created items in 5.0 or 5.1.


mysql-test/r/select.result:
  IN with outer join condition
mysql-test/t/select.test:
  IN with outer join condition
sql/item_cmpfunc.h:
  correct not_null_tables() for IN