Several months ago, Microsoft decided to change up the HttpResponseMessage class. Before, you could simply pass a data type into the constructor, and then return the message
Apparently the new way to do it is detailed here:
http://aspnetwebstack.codeplex.com/discussions/350492
To quote Henrik,
HttpResponseMessage response = new HttpResponseMessage();
response.Content = new ObjectContent(T, myFormatter, "application/some-format");
So basically, one has to create a ObjectContent type, which apparently can be returned as an HttpContent object.