You are not doing anything wrong, IE 6/7 is.

The option that  works in every browser option is using a regular table.

An other option would be to render conditional HTML it's a bit of a hack, but it does work fine.
(it renders a table for IE below 8 and divs for the rest)

Add this on top:

<!-- Because IE < 8 does not support display:table-cell-->
<%  If Request.Browser.Browser = "IE" AND Request.Browser.MajorVersion < 8 Then%>
    <table id="s_wrap_main">
        <tr id="s_wrap_sub">
        <td class="template_style">
<%Else %>
        <div id="s_wrap_main">
        <div id="s_wrap_sub">
        <div class="template_style">   
<% End If%>

And this at the bottom of your skin:

<!-- Because IE < 8 does not support display:table-cell-->
<%  If Request.Browser.Browser = "IE" AND Request.Browser.MajorVersion < 8 Then%>
            </td>
        </tr>
    </table>
<%Else %>
        </div>
    </div>
    </div>
<% End If%>

Because IE7 does not respect the W3C standards.
This is only be an issue if you add content that is wider then the skin itself, in which case you can use one of the wider skin versions, or change the width of the skin itself
So IMO it's a minor issue, since there is a solution for it.
The solution I posted is bit too much of a hack IMO to add to the default skin.