Used fiddler to double-check* this.
Everything I get back from the API is definitely gzip compressed.
For that matter, you can't get a response that isn't compressed; you can only choose between gzip & deflate. Default is gzip.
The API always sets the Content-Encoding response header, so you might be the victim of your library decompressing the response for you.
*Triple-checked: I ran our test cases without setting Accept-Encoding but leaving the gzip decompression code in.
Bare minimum request using the world's worst stackoverflow clone (w/ gzip off):
Header:
GET http://api.stackoverflow.com/0.8/users/242897/comments/160173?sort=votes&order=desc&page=1&pagesize=30&key=iv1qWOxcwkaKg53RJIXh-A HTTP/1.1
User-Agent: WorldsWorstStackOverflowClone
Host: api.stackoverflow.com
Connection: Keep-Alive
Response:
HTTP/1.1 200 OK
Server: nginx
Date: Sun, 13 Jun 2010 13:44:21 GMT
Content-Type: application/json; charset=utf-8
Connection: keep-alive
Cache-Control: private
Content-Encoding: gzip
X-AspNetMvc-Version: 2.0
X-RateLimit-Max: 10000
X-RateLimit-Current: 9473
X-AspNet-Version: 2.0.50727
Content-Length: 625
[gzip'd byte stream]