Hi,
I tried to open user contribution page (with a default user contribution web part) and it was opened correctly as a new iframe in a modal window. I was able to edit contribution as well. Tested on 6.0.15 Here is my code:
// inserted in the editable text web part:
<p>
	<a class="iframe" href="http://<domain>/Examples/Web-parts/User-contributions/Edit-contribution.aspx">User contribution (Iframe)</a></p>
// script in the head section on master page
<link rel="stylesheet" href="http://<domain>/CMSScripts/jquery/colorbox.css" />      
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="http://<domain>/CMSScripts/jquery/jquery-colorbox.js"></script>
<script>
     $(document).ready(function(){
				//Examples of how to assign the ColorBox event to elements
				$(".group1").colorbox({rel:'group1'});
				$(".group2").colorbox({rel:'group2', transition:"fade"});
				$(".group3").colorbox({rel:'group3', transition:"none", width:"75%", height:"75%"});
				$(".group4").colorbox({rel:'group4', slideshow:true});
				$(".ajax").colorbox();
				$(".youtube").colorbox({iframe:true, innerWidth:425, innerHeight:344});
				$(".iframe").colorbox({iframe:true, width:"80%", height:"80%"});
				$(".inline").colorbox({inline:true, width:"50%"});
				$(".callbacks").colorbox({
					onOpen:function(){ alert('onOpen: colorbox is about to open'); },
					onLoad:function(){ alert('onLoad: colorbox has started to load the targeted content'); },
					onComplete:function(){ alert('onComplete: colorbox has displayed the loaded content'); },
					onCleanup:function(){ alert('onCleanup: colorbox has begun the close process'); },
					onClosed:function(){ alert('onClosed: colorbox has completely closed'); }
				});
				
				//Example of preserving a JavaScript event for inline calls.
				$("#click").click(function(){ 
					$('#click').css({"background-color":"#f00", "color":"#fff", "cursor":"inherit"}).text("Open this window again and this message will still be here.");
					return false;
				});
			});
</script>
I hope it helps.
Best regards,
Ivana Tomanickova