A client can tell the server that he can only understand some specific message format in HTTP with following header info:
If the server does not support this media type, he should return a HTTP 406 Status Code (Not Acceptable) with an optional help URI:
406 Not Acceptable
But the Web API returns XML by default. It should be returning a 406 by default...
See also: RESTful Web Services Cookbook by Subbu Allamaraju, O´Reiily, Page 132