I recently set myself the task of making the ecommerce site for the company I work for XHTML 1.1 compliant. I discovered that once you set the compatibility mode in the web.config to 1.0 strict, it was only a short hop up to 1.1, until I hit a snag.
When using web forms, .net automatically inserts the name of the form in the form tag which messes up 1.1. Fortunately I discovered this article and was able to remove the offending name attribute
I'm not sure how many asp.net 2.0 XHTML 1.1 compliant sites there will be out there, but I doubt whether it will be many