It hasn't made it in to the <layer> tag documentation yet, but there is a sample called "Clustering" listed on the "Samples" page.Here are some examples of how to take advantage of the new <clustering> functionality in flex viewer 2.4.Basic example:
<layer label="Simple, same-sized circles (default color green)"
type="feature"
url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/SanFrancisco/311Incidents/MapServer/0"
popupconfig="popups/PopUp_311.xml"
visible="true">
<clustering>
<clustersymbol type="simple"/>
</clustering>
</layer>
The example from the online sample:<layer label="San Franscisco 311" type="feature" url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/SanFrancisco/311Incidents/MapServer/0" popupconfig="popups/PopUp_311.xml" visible="true">
<clustering mingraphiccount="2" sizeinpixels="80">
<clustersymbol type="simple" alpha="1" alphas="0.7,0.8,1" bordercolor="0xFFFFFF" color="0xFF4242" colors="0xFF837E,0xFF4242,0xFF4242" flaremaxcount="10" size="90" sizes="30,60,90" textcolor="0x000000" textsize="14" weights="10,100,200"/>
</clustering>
</layer>