Check out this blog post: [node:15427] – we added a “use Sign In With Twitter” checkbox to your application configuration, with it defaulted to off. “Opt-in” to the behavior by checking the box and saving. While you’re at it, make sure you’re explicitly specifying your oauth_callback on oauth/request_token and sending oauth_verifier on oauth/access_token – you probably are, but doesn’t hurt to make sure.
Thanks!