Hi, i've just properly configured DNN on my local computer (i have't done any change to the source code yet), and i tried to opened the Default.aspx under the site root with VS2005. and here is an error i got when i tried to switch from code view to design view.

ERROR Creating Control: Form

Unable to cast object of type 'System.Web.UI.Design.HTMLIntrisicControlDesigner' to type 'System.Web.UI.Design.ControlDesigner'.

have anyone seen this kind of problem before?

I get the same error.  Apparently the DNN Form control can't be rendered by VS2005.  Is there a reason you're trying to edit the Default.aspx?

Specifically, the issue is that the dnn form control inherits from System.Web.UI.HtmlControls.HtmlForm.  User web controls, it would seem, are expected to inherit from a WebControl, rather than an HtmlControl, so the designer support in VS2005 doesn't work.  It's not a problem with the code, it's just a problem with VS2005 not being able to show it in the designer.  You'll have to a) ignore the error, b)stick to using code view, or c)inplement a designer class (and then submit it to the core project).  There really aren't many options (though it also shouldn't really be causing any actual problems).