As far as I am aware there is no option to render ASP.Net web controls as XML, HTML is the only option. The System.Web.UI.Page method that is responsible for displaying the controls is:

public void RenderControl(
   HtmlTextWriter writer

In most cases it wouldn't make a lot of sense to render controls as XML. What it would make sense to render as XML is the content.

I notice there is talk of Master Templates in VS 2005 I am hoping this will move towards ASP.Net using a front controller instead of a page controller, such as in Maverick.Net. This would facilitate a much cleaner separation in the web presentation layer.