If you get the latest code again, I made a change to help you.
What you can do is add an extra SiteMapDataSource with a custom id like this:
<asp:SiteMapDataSource ID="MySiteMapData" runat="server" ShowStartingNode="false" />
I've added a new setting "DataSourceId" on SiteMenu so you can set it to use your custom data source that way it won't interfere with the one used by the SiteMap page.
<portal:SiteMenu id="SiteMenu1" runat="server" DataSourceId="MySiteMapData" StartingNodeOffset="1"
UseTreeView="false" UseSpanInLinks="true" HideMenuOnSiteMap="false"
Direction="Horizontal" TopLevelOnly="true"></portal:SiteMenu>
Hope it helps,
Joe