Interesting. Looking at Michael O. Church's premise, I wonder if it might be possible to bootstrap a professional organization by (initially) inviting only those engineers who already work at a level we'd consider professional: basically, the false negative rate here would need to be <~5%. One option might be to pick only those who have worked, for 2+ years, at a level above "terminal" levels:
http://www.quora.com/Engineering-LaddersSo the initial invite would be for E6s+ at a few organizations (Google, Facebook, others?). Those people would know others they can vouch for, at either the "senior" level (the E6+ level) or at a "junior" level, but who are on a trajectory to make the "senior" level, eventually. At some point, this group would need to establish either which schools, or which certifications (i.e. MOOC programs) qualify someone for "aspiring junior" or similar roles, much like the bar. Or it could be entirely "project based", i.e. you show the organization some work you've accomplished, and the source code, in order to qualify. I haven't fully thought this through, so I don't really know how to get this right.
Given enough cachet, this should allow SDEs to form partnerships, with relatively little "reputation verification" required from their clients, much like how for most lawyers and doctors, people don't really care which one they go to (at least at first), just that they're seeing someone qualified.