You theoretically could offer snail mail as well - there are services that translate from email to letters for you.
A left field comment -- Right now, this is a developer-orientated service. You could market it to consumers - e.g. This is my messagepub address, send your messages there. Then the consumer as a receiver gets to decide on how messages get routed to them.
A more difficult play, but it would be interesting to see. The revenue part might be more difficult, but perhaps there are ways around this.
It would also have the advantage that applications don't need to know and maintain everything about you (e.g email, phone, cell, twitter, etc, etc). Even in this model it looks like every app is responsible for maintaining all the required info.
The consumer could just maintain it in one place... Sort of like a Grand Central for the Web.