Someone make textcastr.com please. Yes, the domain is available.
The service is simple. A user creates an account at textcastr.com, enters their user names at twitter, pownce, facebook, etc. Textcastr.com provides their SMS short code to the user... messages sent to the short code are then relayed to the appropriate services.
The way you get the message to the available services will depend on the service of course. Twitter and Facebook have APIs, Pownce? No one knows yet.
There. Take it. And be sure to let me know when I can sign up.
