JQuery :: Passing Value To Hidden Form Field?

I have a function that gets the value of a clicked on <span> like this:

function getSelectedValue(id) {
return $("#" + id).find("dt a span.value").html();
}

and sets the value of a second span like this:

$("#state").html("value: " + getSelectedValue("state"));

When I echo that second span, I get the selected value from the first span. But I need to get that value into a hidden html form field, so I can pass it along with the $_POST array. How do I do this?