I've create a custom list with schema.xml, custom DefaultTemplates.ascx [copy of the DefaultTemplates.ascx render template "ListForm"], and custom variations of the NewForm.aspx, EditForm.aspx, and DispForm.aspx. I've hooked up the forms in the schema.xml and clicking "New" on the default AllItems view does go to NewPage.aspx as I would expect.
I have hidden the ListForm WebPart and added a DataForm WebPart using xslt in the section. My problem is that any changes I make to the XSL don't seem to show up when viewing the NewForm.aspx. I can make changes to the custom DefaultTemplates.ascx file and I'll see those. I'm not sure why my changes aren't showing up, but I'm guessing the fact that changes to custom version of DefaultTemplates.ascx is suggesting something. I jsut don't what it means yet.
Btw, I have all of the files packaged in a Visual Studio 2008 project and packaging it up into a wps on build. All the files get added in the wsp and seemingly deployed correctly (they get copied correctly to disk and all list template and list instance code works).
One other piece of information, I can open SharePoint Designer and make a copy of my NewForm.aspx, but it only contains the normal ListForm WebPart; no sign of my DataForm WebPart with XSL. I can hide the ListForm WebPart and a DataForm WebPart to it and that works as typically expected.
I know this is a bit of information, but if anyone has any idea about what might be happening I will be greatful.