It's trivial to build an APP that generates an RSS stream for an arbitrary twitter account with their API, and I'm sure someone will soon. But the point is that they are setting a new standard for what can be done streams of structured data, and hopefully more sites will move away from RSS into cool APIs.
Of course, for now at least, there's no standard to this kind of API, but I think that people will eventually figure out what works and converge to it (as is the case with OAuth).