From Twitter’s just released guidelines surrounding the updated API rules:
API v1.1 will support JSON only. We’ve been hinting at this for some time now, first dropping XML support on theStreaming API and more recently on the trends API. We’ve chosen to throw our support behind the JSON format shared across the platform. Consequently, we’ve decided to discontinue support for XML, Atom, and RSS, which are infrequently used today. For historical context, when we originally built the API all major languages did not have performant, well vetted libraries supporting JSON — today they do.
I’m deathly afraid that RSS is dying a slow death by a thousand small cuts. It’s happening all over the place and we will miss it when it’s gone.