Yes, you will see some methods for posts using xml and some json.  I like json better as it removes a dependency on xmlbuilder but I wasn't able to get it working at the time so I just used xml for some.  For the most part I was trying to keep the majority doing json goes in and json comes back rather than having a mix.  I think I would have to see an example of using json for the content_type to see if it increases complexity or if understandability declines (granted we could hide this form the user) then I would say just stick with xml for posts.  Do you guys have a preference?