Having a collection of servers that act as transaction clearing houses for people would probably work but it seems to break the whole p2p idea of the system.
Why? E-Mail and
Jabber work the same way. Everyone
can run their own server, and many do, but most users prefer to use someone else's server(s).
Edit: Even more closely related: Kazaa and Gnutella work the same way, too. You connect to a
supernode that then searches for files on your behalf.