I've added 2 custom picklists to the Opportunities page. One of the picklists is a standard picklist with static values, and the other is a dependent picklist whose content is based on the value of the selected item in the the first picklist.

I need to find a way to force the user to select all of the items that are presented in the dependent picklist.

Can this be done with validation rules, or would it require a custom S-control or a trigger?