This would require some sort of single-sign-on, or capabilities system. Would love to see capabilities-based security for web services, actually. "Here is a token that grants permission to service X to perform action Y for duration Z." Can OpenID and ilk do this?
Anyway, apps that don't need security should be as you describe already.
This is pretty much exactly what OAuth does - enables the user to authorize a web service to access another one on their behalf, with per-app, per-user constraints.
Anyway, apps that don't need security should be as you describe already.