I'm using a valid key and secret, and the app successfully gets an xauth token. When I try to send a test tweet, I see the following in the console:
2010-08-20 09:00:33.769 XAuthTwitterEngineDemo[7934:207] Twitter request failed: 1AC8A041-B09F-4F79-8ACA-1AB8A7F82FB8 with error:Error Domain=HTTP Code=401 "The operation couldn’t be completed. (HTTP error 401.)"