Robert Bogue has an article on DevX explaining the wrapping of ASP.NET Web controls inside a SharePoint web part. Personally, I would not construct my web parts this way however. I don't know, but what is the reason not to use the CreateChildControls() and the EnsureChildControls in combination with the RenderWebPart()? I could be missing out something, but to me that is the obvious way to go if you do all the hard work inside the web part yourself. Much easier of course if you make use of the SmartPart and the technique of the ASP.NET user controls.