Re: Are We Wasting Our Time?
If you want to go with WP + BP I would seriously just invest time in a small test setup. Get comfortable with the coding practices. Make a small purposeful plugin from scratch. Learn what makes wp different from wpmu. Anticipate change in software, lots and often, so don’t build on current solution, but look at the roadmap of both wpmu and bp.
Consider that wp 3 is slated to be a media release and I would think it is moving more and more to cms. Waiting on that to mature would possibly be a wise decision. That doesn’t mean you should not start. Just start with what is relevant and mature now.
Consider also if wp and bp are scalable enough. You might want to look at more powerful cms like Drupal. Drupal has a steep and painful learning curve, but it is incredibly scalable and would go the distance in what you are looking for. I don’t use Drupal myself anymore, for the reason that developing for it is slow. However, it might be a better fit.
Also you have to consider that your idea totally tanks, put yourself in the shoes of a critical viewer and bash the heck out of your own idea. Write it down, take it serious and find solutions for it. The fact is, that it is not a new idea, it just hasn’t been done for a good reason. Find that reason and argue it.