I left out a bunch of stuff when writing about replacing Twitter yesterday.
A big piece is this: a client developer would probably have to write and run their own web services. (And would probably have to have subscription pricing so they have recurring revenue so they can afford it.)
The services would provide a few things at a minimum:
Hosting for published statuses.
A content service that reads the feeds, so that you don’t have a zillion clients all polling. (Not rocket science. As done by NewsGator, Google Reader, Bloglines, Fever, etc.)
Push notifications for new statuses.
An API for the clients to talk efficiently to the content service (to be able to get just what’s new, for instance).
It’s work, and it takes money and time.
At any rate — even if I’m wrong in every single way about how to replace Twitter, I would still be very nervous as a Twitter client developer, and I would absolutely start talking with other developers about how to move on.