If Posterous wants to be a STAND UP company, they'll post information and a tool(not just XML or API) on how to migrate away FROM Posterous. Then they can go and write importers for other sites. I'm sick of all these companies talking shit but not being able to back it up when they say they're looking out for the user!
Is it really a web app company's responsibility to actually write importers for competitors? It's certainly a responsible practice to document the API well, but you're basically asking for your landlord to pay for your moving van when you move out.