select boxes

I am trying to understand the best way to apply javascript to two
select boxes. If the user changes the selection in box A, a given
option in box B would dynamically become selected.

Does rails provide help with tasks like this, or would I need to
manually create a javascript function to handle this kind of thing in
the view?

Whatever you do boils down to writing an onchange handler for A. You can do this in a railsish way: you can use the update page to let you write that bit of javascript using rjs, but I would contend that there isn't a huge amount of point doing it that way. With a smattering of prototype, javascript really isn't that bad.