If you have a HeaderTemplate or a FooterTemplate defined, you could add any HtmlControl or ServerControl inside of either of them and then programatically show/hide it in the codebehind.
<asp:Repeater id="Repeater1" runat="server" OnItemDataBound="">
<HeaderTemplate>
<h1>My Repeater Data</h1>
<div id="NoRecords" runat="server" visible="false">
No records are available.
</div>
</HeaderTemplate>
<ItemTemplate>
...
</ItemTemplate>
</asp:Repeater>
Here's the code behind
protected void Repeater1_ItemDataBound(object sender, RepeaterItemEventArgs e)
{
if (Repeater1.Items.Count < 1)
{
if (e.Item.ItemType == ListItemType.Header)
{
HtmlGenericControl noRecordsDiv = (e.Item.FindControl("NoRecords") as HtmlGenericControl);
if (noRecordsDiv != null) {
noRecordsDiv.Visible = true;
}
}
}
}