Wave will be the killer app by itself if (and only if) there will be free and open source versions of the Wave frontend and server they use to present you their own Wave system. In that case you and your company can each have their own server, data silo and therefore enforcable data security without the *need* to be a silo - just drag in an external user to share and the federation magic will open up the silo for just this wave or wavelet (that is, a sub-wave).
Not being able to decide what data stays with me and what may be seen by public (=Google) servers is the one key reason that "cloud advocates" have no chance in any data-conscious company. Wave (the protocol) makes it possible to have your own data silo that is optionally able to exchange the data with any(!) other Wave server and their users. Add a policy to that decision and there you go with an email-killer in corporations.
Writing your own POP3-client is reasonably possible - replicating the Wave software as is in beta now requires a complete Google-team to do. Read as: It's not easy at all. And anything less usable than the Google Wave frontend will be ditched by all people but techies.