Whether I use json or rest, the result is always 200 with an empty body, instead of either an XML feed or a JSON object.

Here is the response:

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/json;charset=utf-8
Content-Length: 0
Date: Thu, 06 Nov 2008 06:44:51 GMT

I've seen other reports of developers experiencing the same problem here:
http://hi5.com/friend/group/2364084--Hi5%2BPlatform%2BDevelopment--topics-html