Noodling around, discovered the answer. Fill in the "default value" field with your list, separated by semi-colons. For example, "C Coding $80;Perl Coding $70;SQL Dev $50;Database Design $120" and then you'll get a drop-down when you add the attribute to your task.
I can't immediately see any way to provide a "list of values." What I'm hoping for is a way to implement billing categories (coding, support, db mgmt, etc.) where the description can provide an hourly rate for that particular function (more complicated stuff gets a higher rate). "List of values" seemed to me to indicate that I'd provide a list which would be selectable in a drop-down or otherwise.