Creating a new form in a list by copying an existing form (in code)

To copy an existing form in a WSS list, and to add a new content editor web part to that list, use the SPFile class (not the SPForm collection of the list):

//Copy the NewForm.aspx to NewForm_new.aspx
string formPath = "Lists/ClientTest/NewForm.aspx";
string newformPath = formPath.Replace(".aspx", "_new.aspx");
SPFile form = web.GetFile(formPath);
form.CopyTo(newformPath, false);

//Get a reference to the new form, and create a new content editor web part containing onload JavaScript
SPFile copiedForm = web.GetFile(newformPath);
SPLimitedWebPartManager wpManager = copiedForm.GetLimitedWebPartManager(PersonalizationScope.Shared);

//Create the new web part
ContentEditorWebPart wpContent = new ContentEditorWebPart();
XmlDocument xmlContent = new XmlDocument();
XmlElement el = xmlContent.CreateElement("El");
el.InnerText = "";
wpContent.Content = el;
wpManager.AddWebPart(wpContent, "Main", 0);

The JavaScript adds an onload handler to the page, allowing for behaviours to modify the content of the standard list form for example.