It turns out the RadioButtonList is rendered as an HTML Table element, with the table getting the corresponding id of the server control and the radio buttons getting id values that had an increment on the end. Their name attribute, which groups them, was the same as the RadioButtonList id.
Simple example: you have a RadioButtonList you give an id of myList. ASP.NET 2.0 renders a table with the id attribute of "myList." You'll have your radio buttons inside, named myList_0, myList_1, and so on. These radio buttons, rendered as getElementsByTagName("input") to get your radio buttons. Test to see which is selected and you're all done. The following function encapsulates that logic, and we've begun reusing it all over the place. The radioList argument should just be the ClientId of your RadioButtonList:
var options = radioList.getElementsByTagName('input');
var opt = options[i];