Growing up, I remember having to show resort ID to ride. Then, the practice seemed to fade away. First, ID checks became sporadic, then non-existent.
Technically, I think the rule is still in effect, but it is not enforced. Think about it: WDW advertises the 'perk' of staying on site as 'use of WDW transportation' - implying that guests without a Park Hopping option or Resort ID are not permitted to ride.