Because of namespace issues with DOMXPath, if you are using fillInFormFilter with content_type = xml (which people have been trying to do, in order to output xhtml-compliant code), the filter will fail, reporting "No form found in this page".
The xml option was added as a result of ticket #512.
ReeD figured out the source of the problem and proposed a solution in this thread: http://www.symfony-project.com/forum/index.php/t/6044/
The solution is to explicitly load a new namespace and use that in all queries, since it is the default namespace which is causing the DOMXPath queries to fail. Examples and comments on the PHP DOMXPath documentation page also seem to show this to be the solution that is commonly used for xhtml.
I've attached the proposed solution by ReeD as a patch. I've also tested it and found it to work.