In Spring MVC, how can I set the mime type header when using @ResponseBody

Use ResponseEntity instead of ResponseBody. This way you have access to the response headers and you can set the appropiate content type. According to the Spring docs:

The HttpEntity is similar to
@RequestBody and @ResponseBody.
Besides getting access to the request
and response body, HttpEntity (and the
response-specific subclass
ResponseEntity) also allows access to
the request and response headers

The code will look like:

@RequestMapping(method=RequestMethod.GET, value="/fooBar")
public ResponseEntity<String> fooBar2() {
    String json = "jsonResponse";
    HttpHeaders responseHeaders = new HttpHeaders();
    responseHeaders.setContentType(MediaType.APPLICATION_JSON);
    return new ResponseEntity<String>(json, responseHeaders, HttpStatus.CREATED);
}