Server side XForms form validation and integration into ASP.NET
I'm probably missing something but...
The way I've done this sort of thing in the pass is to create a schema for form declaration & an xslt to convert that xml to html. You can then add all sorts of validation, based on the xml schema you've designed. What I am usually missing is the form designer though.
You can use XHTML for the forms, or like Simon suggested, a custom language as an abstraction layer. The application can load the form at runtime, transform/style it and serve it to the user. You can edit the form and make changes without the need to recompile/redeploy.
For validation you can use Schematron, this can also be easily edited at runtime using XML-aware tools.
You can use Javascript to produce XML data in the browser, or you can transform application/x-www-form-urlencoded to XML on the server (that is what I do on this page).