Hey Steve, check out the following code.  I didn't use a template
control, but a CompositeControl instead.  You can lookup the ITemplate
interface and the InstantiateIn method to figure out how to use it.  It
would be cool to make the helptext layer a template to provide more
than just text for help.

Anyway, you can copy the following code into a new windows class
project and compile it into a dll.  You can then use the following tag
to embedd the control in the page.

<cc1:Overlay runat="server" id="ov1" width="100px" Image="Lit.gif"
HelpImage="Normal.gif" HelpText="My Help Text" />

Here's the code:

/// <summary>
   /// Michael Hamrah from Steve Barker
   /// Sorry, it's not too pretty!
   /// The following code shows
   /// 1) Overriding the AddAttributesToRender method for doing
mouseovers
   /// 2) Overriding the Render method to place controls next to
eachother
   /// 3) Overriding the CreateChildControls method to add controls
during runtime
   /// 4) Using the Page.ClientScript property to render Javascript
   /// </summary>
   public class Overlay : CompositeControl
   {
       private string _img = string.Empty;
       private string _helpText = string.Empty;
       private string _helpImg = string.Empty;

       public Overlay()
       {

       }
       protected override HtmlTextWriterTag TagKey
       {
           get
           {
               return HtmlTextWriterTag.Span;
           }
       }
       public string HelpImage
       {
           get { return _helpImg; }
           set { _helpImg = value; }
       }
       public string Image
       {
           get { return _img; }
           set { _img = value; }
       }
       public string HelpText
       {
           get { return _helpText; }
           set { _helpText = value; }
       }
       //add image to control
       protected override void CreateChildControls()
       {
           Image img = new Image();
           img.ID = "litImg";
           img.ImageUrl = this.Image;
           this.Controls.Add(img);

           StringBuilder sb = new StringBuilder();
           sb.Append("<script type=\"text/javascript\"> function
hideLayer(layer) { document.all[layer].style.visibility = \"hidden\"; }
function showLayer(layer) { document.all[layer].style.visibility =
\"visible\"; } </script>");

if(Page.ClientScript.IsClientScriptBlockRegistered("helpJS") == false)

Page.ClientScript.RegisterClientScriptBlock(typeof(string), "helpJS",
sb.ToString());

           if
(Page.ClientScript.IsStartupScriptRegistered(this.ClientID) == false)
               Page.ClientScript.RegisterStartupScript(typeof(string),
this.ClientID, "<script type=\"text/javascript\">hideLayer('" +
this.ClientID + "x');</script>");

           base.CreateChildControls();
       }
       //add attributes to span tag
       protected override void AddAttributesToRender(HtmlTextWriter
writer)
       {

           StringBuilder sb = new StringBuilder();

           sb.Append("showLayer('" + this.ClientID + "x')");
           if (this.Image != string.Empty)
               sb.Append(";" + this.ID + "_litImg.src='" + this.Image
+ "'");

           sb.Append(";");
           this.Attributes.Add("onmouseover", sb.ToString());

           sb = new StringBuilder();
           sb.Append("hideLayer('" + this.ClientID + "x')");
           if(this.HelpImage != string.Empty)
               sb.Append(";" + this.ID + "_litImg.src='" +
this.HelpImage + "'");

           sb.Append(";");
           this.Attributes.Add("onmouseout", sb.ToString());

           base.AddAttributesToRender(writer);
       }
       //Need to add layer span tag after real span tag so override
this function.
       protected override void Render(HtmlTextWriter writer)
       {
           base.Render(writer);

           StringBuilder sb = new StringBuilder();
           sb.Append("<script
type=\"text/javascript\">document.write(\"<span ");
           sb.Append(" style=\\\"position:absolute;");
           if (this.Width != 0)
               sb.Append("width:" + this.Width + "\\\"");
           sb.Append(" id=\\\"" + this.ClientID + "x\\\">");
           sb.Append(this.HelpText);
           sb.Append("</span>\");</script>");
           writer.Write(sb.ToString());

       }
   }