One of the images we use to describe IPD is this:
When John testified at an NCARB hearing earlier this year, someone asked him to identify the architect and the builder.
As he explained, it really depends "when". At each point in the process, the group that is best qualified to steer and direct does so. The rest of the team has to trust that group or individual to steer everyone in the correct direction, at the appropriate pace.
All of the process maps that we're seeing (both internally & externally created) really mirror that philosophy..no surprise there. It really gets back to "who is the most qualified to steer at that moment in time".