As far as I know there isn't something built in that can do this, but it is possible to do this using JQuery, and having a function that attached a JQuery event listener to the dropdown when the page loads. We've done that on a few forms for dropdowns with "other, please enter" that displays a field that we hide if it's selected, so they can enter the other option.

:) 

Yup currently the best options is to do this client side with some JQuery

Don't spose you have some examples? I'm wanting to have similar functionality on a SimpleMediaPicker control I'm using for a dashboard admin task. (eg visually show which of the mediapickers have changed there values from the current stored one)

Here's a solution I came up with for anyone stumbling accross this.

(it's quite specific to the media picker...)

So from the base code [http://umbraco.codeplex.com/SourceControl/changeset/view/751c9aa11bbd#components%2feditorControls%2fmediapicker%2fMediaPicker.js]

We can use the .show() .hide() actions called on the $("#" + this._mediaTitleClientID).parent()

By extending the show/hide method to chain our own changed event too, something like

<script language="javascript">
//<![CDATA[
$(document).ready(function(){
    //alert("here");
    
    //$.each(["show", "toggle", "toggleClass", "addClass", "removeClass"], function(){
    $.each(["show", "hide"], function(){
        var _oldFn = $.fn[this];
        $.fn[this] = function(){
            //var hidden = this.find(":hidden").add(this.filter(":hidden"));
            var h = this.find('span[id*=SimpleMediaPicker1_title]');
            var result = _oldFn.apply(this, arguments);
            //hidden.filter(":visible").each(function(){
            h.each(function(){
                $(this).triggerHandler("changed"); //No bubbling
            });
            return result;
        }
    });
 
    $('span[id*=SimpleMediaPicker1_title]').bind("changed", function(){
        //ignore the initial set up of values when first entering the page
        if ( $(this).data("initialValue") != undefined){
            if ($(this).is(":visible"))
            {
                if ($(this).text() != $(this).data("initialValue")){                
                    $(this).parent().parent().css('background-color', 'bisque');
                }
                else
                {
                    $(this).parent().parent().css('background-color', '');
                }
            }
            else
            {
                //but now the title is hidden as the delete function simply hides and doesn't change the text
                if ($(this).text() != '' && $(this).data("initialValue") != '' ){
                    $(this).parent().parent().css('background-color', 'bisque');
                }
                else
                {
                    $(this).parent().parent().css('background-color', '');
                }
            }
        }
    });  
    
    //need to store all the initial values
    $('span[id*=SimpleMediaPicker1_title]').each(function(){
            $(this).data('initialValue', $(this).text());
    });
   
    // extending the show function based on this solution
    //http://stackoverflow.com/questions/1225102/jquery-event-to-trigger-action-when-a-div-is-made-visible
});
//]]>
</script>

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