The problem seems to be that #messages is a parent of #sidbear
You just need to make a small adjustment to the HTML like so
<div id="header"> </div>
<div id="top_position"> </div>
<div id="container">
<div id="messages" class="span_9_of_12 element">
<div class="msg_cont">
<div class="msg">
<p>Hi!</p>
</div>
<div class="msg">
<p>Hi!</p>
</div>
<div class="msg">
<p>Hi!</p>
</div>
</div>
</div>
<div id="sidebar" class="span_3_of_12 element" style="float:right;">
<div class="sidebar_cont">
<div class="sidebar_el">
<p>Hi again!</p>
</div>
<div class="sidebar_el">
<p>Hi again!</p>
</div>
</div>
</div>
</div>
See this updated fiddle