Something like this should do the trick (you need to use another plug in to select your plug in):

[GuiPlugIn(Area = PlugInArea.EditPanel)]
public class SelectYourEditPanelPlugIn : ICustomPlugInLoader
{
    public PlugInDescriptor[] List()
    {
        // hook LoadComplete-event on EditPanel page
        EditPanel editPanel = HttpContext.Current.Handler as EditPanel;

        if (null != editPanel)
        {
            editPanel.LoadComplete += new EventHandler(editPanel_LoadComplete);
        }

        //Never return a plugin - we don't want to add tabs.
        return new PlugInDescriptor[0] { };
    }

    protected void editPanel_LoadComplete(object sender, EventArgs e)
    {
        // find the TabStrip with id = "actionTab"
        TabStrip actionTabStrip = this.FindControl<TabStrip>(sender as Control, "actionTab");

        //Find the tab
        if (actionTabStrip != null)
        {
            int yourPlugInID = PlugInDescriptor.Load(typeof(YourPlugIn)).ID;

            foreach (var item in actionTabStrip.Controls)
            {
                Tab actionTab = item as Tab;
                if (actionTab != null && actionTab.PlugInID == yourPlugInID)
                {
                    actionTabStrip.SetSelectedTab(actionTab.ID);
                }
            }
        }
    }

    // try to locate control of type T with ID==id
    // recurses into each childcontrol until first match is found
    protected T FindControl<T>(Control control, string id) where T : Control
    {
        T controlTest = control as T;

        if (null != controlTest && (null == id || controlTest.ID.Equals(id)))
            return controlTest;

        foreach (Control c in control.Controls)
        {
            controlTest = FindControl<T>(c, id);
            if (null != controlTest)
                return controlTest;
        }

        return null;
    }

    protected T FindControl<T>(Control control) where T : Control
    {
        return FindControl<T>(control, null);
    }

}

See http://labs.episerver.com/en/Blogs/Allan/Dates/2009/1/Neat-Trick-Modifying-Edit-Mode-Tabs/ for more info on where I got the inspiration from 

Cheers mate worked like a charm.

had some problem when trying to move off the tab although moved the call into editPanel_PreLoad instead, so now it only default's to the tab if selected tab is empty.

Yeah, I forgot to add that you should check something in the querystring to see if you should select or not :)