The best I think you'll be able to do is to hack the web form that VS.NET uses as a template for new forms. For VS.NET 2003 and C# as a language, the template used for the new web form can be found at:
C:\Program Files\Microsoft Visual Studio .NET 2003\VC#\VC#Wizards\CSharpAddWebFormWiz\Templates\1033\WebForm1.aspx
You will see the docType at the top as you would with a normal page, and you can change it to whatever you like.