Prototype and application/json

I have a feeling I am doing something wrong here (the great thing with Rails is that when it gets difficult you can generally assume that to be the case!), so any comments are appreciated.

I have a link on a page that makes an XHR. I'd like to return a JSON object to the page which can then be eval'd and various bits of data used. I set up the routing to make sure the format came through as JSON (http://xxx/xxx/xx.json) using :format, and the controller returns the JSON object. The problem is that that response is sent as 'application/json', which is not a default accept header (so I end up with a 406 response).

Obviously I could adjust the response header to include that MIME type, but my guess is that I am doing something stupid here and have missed something - any ideas appreciated!