This is because PostgreSQL 9.1 added the feature of simple checking of functional dependencies for GROUP BY. The manual of 9.1 explains quite well when PostgreSQL considers there to be a functional dependency.
"When GROUP BY is present, it is not valid for the SELECT list expressions to refer to ungrouped columns except within aggregate functions or if the ungrouped column is functionally dependent on the grouped columns, since there would otherwise be more than one possible value to return for an ungrouped column. A functional dependency exists if the grouped columns (or a subset thereof) are the primary key of the table containing the ungrouped column."
I completely agree with documentation.
But my case shows that "not valid" expression which refers to column which is ungrouped still works in 9.1.
