Scratching my head about this. In the rendered HTML for the code below, the btnEdit (in the GridView) has the correct Javascript in the onclick parameter (onclick="javascript:WebForm_DoPostBack..."). The btnAddNew has no onclick handler at all. Why? There is no compilation or runtime error, and the page uses a master page that has the Form tag..
<ContentTemplate>
<asp:ImageButton ID="btnAddNew" SkinID="btnAddNew" runat="server"
PostBackUrl='<%# "EditUser.aspx?action="+Constants.actionAdd %>' />
<asp:GridView ID="UserGridView"
runat="server"
DataKeyNames="UserId"
>
<Columns>
<asp:TemplateField
<ItemTemplate>
<asp:ImageButton id="btnEdit" SkinID="btnEdit" runat="server"
PostBackUrl='<%# Eval("UserId", "EditUser.aspx?
action="+Constants.actionEdit+"&uid={0}") %>' />
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
it looks like you don't need data binding tag (<%#) for the btnAddNew button. So you can assign this property at server side :
btnAddNew.PostBackUrl = "EditUser.aspx?action=" + Constants.actionAdd;