This answer is to just confirm that the answer by axtavt works. It took me a minute to realize what he was suggesting, so I thought I'd post a code-snippet to help out anyone coming along behind me. Kudos go to him, though! :)

MyController.java

@Controller
public class MyController {

    @RequestMapping( method=RequestMethod.GET, value="/mainView" )
    public ModelAndView getMainView( ... ) {        
        /* do all your normal stuff here to build your primary NON-ajax view
         * in the same way you always do
         */             
    }

    /* this is the conroller's part of the magic; I'm just using a simple GET but you
     * could just as easily do a POST here, obviously
     */
    @RequestMapping( method=RequestMethod.GET, value="/subView" )
    public ModelAndView getSubView( Model model ) {
        model.addAttribute( "user", "Joe Dirt" );
        model.addAttribute( "time", new Date() );
        return new ModelAndView( "subView" );
    }

}

mainView.jsp

(...)

<script src="http://code.jquery.com/jquery-1.7.1.min.js"></script>
<script type="text/javascript">
    function doAjaxPost() {

        $.ajax({
            type: "GET",
            url: "subView",
            success: function(response) {
                $("#subViewDiv").html( response );
            }
        });
    }
</script>
<input type="button" value="GO!" onclick="doAjaxPost();" />
<div id="subViewDiv"></div>

(...)

subView.jsp

(...)

<h3>
    User Access Details
</h3>

<p>
    ${user} accessed the system on ${time}
</p>

(...)

And that's it! A thing of beauty; up to now, doing AJAX in Spring has been a huge pain... parsing big @ResponseBody's, building huge sets of HTML by concatenating stuff in JS... ugh... I can't believe how simple and awesome this approach is -- and wasn't aware of it until just now! :)

You can put this piece of page into a separate JSP and return a ModelAndView pointing to it from your method. There are no difference between AJAX and non-AJAX calls from that point.