The thing that I think is lost here is the core of Instagram. Systrom has stated repeatedly that the filters aren't the secret sauce of Instagram. It's the speed of upload, distribution model (able to send to several networks simultaneously), and user interface. I don't think simply adding filters is going to do much for Twitter. Think Facebook Camera (which I believe has filters as well). When you say filtered pictures, no one thinks of Facebook Camera, even though it has filters built in. Not really sure what Twitter's trying to accomplish with this one.