The symfony default _format which is used to determine the right template should be aware of the HTTP Accept header(http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html)

This shouldn't effect the default behavior like it is right now for websites, but if you're building an (REST[FUL]) API this would be a really great improvement.

As far as I discovered it now, the format is determined here: [...]/symfony/src/Symfony/Component/HttpFoundation/Request.php:833

What do you think?