IMO this is really what this developer payment scheme is all about (based on the discussions with Caldwell, et al on app.net). As a twitter clone it is sufficient (minus the network effect), but it is really a messaging and identity platform waiting to be defined by the applications written on top of it. Caldwell alludes to this near the end of the OP.
This sentiment is common, but I don't see what makes an open application bus useful.