Hi elmo,
Unfortunatelly you cannot use ButtonField with ConfirmButtonExtender without
extra coding, insead i'd recommend using templatefield:
<asp:GridView runat="server" ID="gv" AutoGenerateColumns="false">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:Button runat="server" ID="btnDelete" Text="Delete"
CommandName="Delete" />
<ajaxtoolkit:confirmbuttonextender id="cbe" runat="server"
targetcontrolid="btnDelete"
confirmtext="Are you sure you want to delete this record?" />
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
or even better, why not just using templatefield with a button (no
confirmbuttonextender needed):
<asp:GridView runat="server" ID="gv" AutoGenerateColumns="false">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:Button runat="server" ID="btnDelete"
Text="Delete"
CommandName="Delete" OnClientClick="return
confirm('really?');" />
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>