Mark,
You should be able to modify your skin for this.  Both the login and user control are skin objects.  You just need to add something to show a link to a cart in the same place and style it.  You could also wrap it in a Is.Authenticated block so it only shows to registered/logged in members.

Finally. some of the store provide a skin object for this as well which might give you more info about qty and value in the cart at any given time for that member.

Roughly speaking (based on the Gravity Skin):

         <div id="login">
   % If Not Request.IsAuthenticated Then%><A href="https://mysite.com/mycart.aspx><img src="<% =SkinPath%>images/cart.gif /></a><% End If %>
            <dnn:USER ID="dnnUser" runat="server" LegacyMode="false" /> 
            <dnn:LOGIN ID="dnnLogin" CssClass="LoginLink" runat="server" LegacyMode="false" />
       </div><!--/login-->

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Steven Webster
Manager, Community Platform
F5 Networks, DevCentral